Fennel seed side effects


10 Science-Based Benefits of Fennel and Fennel Seeds

Foeniculum vulgare, commonly known as fennel, is a flavorful culinary herb and medicinal plant.

Fennel plants are green and white, with feathery leaves and yellow flowers.

Both the crunchy bulb and the seeds of the fennel plant have a mild, licorice-like flavor. Yet, the flavor of the seeds is more potent due to their powerful essential oils.

Aside from its many culinary uses, fennel and its seeds offer a wide array of health benefits and may provide antioxidant, anti-inflammatory, and antibacterial effects.

Here are 10 benefits of fennel and fennel seeds, all based on science.

1. Highly nutritious

Both fennel and its seeds are packed with nutrients. Here’s the nutrition for 1 cup (87 grams) of raw fennel bulb and 1 tablespoon (6 grams) of dried fennel seeds (1):

Fresh fennel bulb Dried fennel seeds
Calories 27 20
Fiber 3 grams 2 grams
Vitamin C 12% of the RDI 1% of the RDI
Calcium 3% of the RDI 5% of the RDI
Iron 4% of the RDI 6% of the RDI
Magnesium 4% of the RDI 5% of the RDI
Potassium 8% of the RDI 2% of the RDI
Manganese 7% of the RDI 17% of the RDI

As you can see, both fennel and fennel seeds are low in calories but provide many important nutrients.

Fresh fennel bulb is a good source of vitamin C, a water-soluble vitamin critical for immune health, tissue repair, and collagen synthesis (2).

Vitamin C also acts as a potent antioxidant in your body, protecting against cellular damage caused by unstable molecules called free radicals (3).

Both the bulb and seeds contain the mineral manganese, which is important for enzyme activation, metabolism, cellular protection, bone development, blood sugar regulation, and wound healing (4).

Aside from manganese, fennel and its seeds contain other minerals vital to bone health, including potassium, magnesium, and calcium (5).

Summary Fennel and fennel seeds provide important nutrients, such as vitamin C, calcium, magnesium, potassium, and manganese.

2. Contain powerful plant compounds

Perhaps the most impressive benefits of fennel and fennel seeds come from the antioxidants and potent plant compounds they contain.

Essential oil of the plant has been shown to contain more than 87 volatile compounds, including the polyphenol antioxidants rosmarinic acid, chlorogenic acid, quercetin, and apigenin (6).

Polyphenol antioxidants are potent anti-inflammatory agents that have powerful effects on your health.

Studies suggest that people who follow diets rich in these antioxidants have a lower risk of chronic conditions like heart disease, obesity, cancer, neurological diseases, and type 2 diabetes (7).

What’s more, over 28 compounds have been identified in fennel seeds, including anethole, fenchone, methyl chavicol, and limonene.

Animal and test-tube studies note that the organic compoundanethole has anticancer, antimicrobial, antiviral, and anti-inflammatory properties (8).

Finally, the plant compound limonene helps combat free radicals and has been shown to protect rat cells from damage caused by certain chronic diseases (9, 10).

Summary All parts of the fennel plant are rich in powerful antioxidants like chlorogenic acid, limonene, and quercetin — all of which may benefit health.

3. Fennel seeds may suppress appetite

Fennel seeds may not only add depth and flavor to your recipes but also help curb appetite.

A study in 9 healthy women demonstrated that those who drank 8.5 ounces (250 ml) of tea made with 2 grams of fennel seeds before eating lunch felt significantly less hungry and consumed fewer calories during the meal than those who drank a placebo tea (11).

Anethole, a major component of fennel essential oil, may be behind the appetite-suppressing qualities of the plant.

That said, another study in 47 women found that those who supplemented with 300 mg of fennel extract daily for 12 weeks gained a small amount of weight, compared to a placebo group. They also did not experience reduced appetite (12).

Research in this area is conflicting, and more studies are needed to fully understand the potential appetite-suppressing properties of fennel.

Summary Fennel seeds may reduce appetite, yet current study results are conflicting. Thus, more research is needed.

4. Can benefit heart health

Eating fennel and its seeds may benefit heart health in a number of ways, as they’re packed with fiber — a nutrient shown to reduce certain heart disease risk factors like high cholesterol.

A 1-cup (87-grams) serving of raw fennel bulb packs 3 grams of fiber — 11% of the Daily Reference Value (DRV).

Diets high in fiber have been linked to a reduced risk of heart disease. A review of 22 studies associated a greater dietary fiber intake with a lower risk of heart disease. For every additional 7 grams of fiber consumed per day, heart disease risk decreased by 9% (13).

Fennel and its seeds also contain nutrients like magnesium, potassium, and calcium, which play important roles in keeping your heart healthy (14).

For example, including rich sources of potassium in your diet may help reduce high blood pressure, a risk factor for heart disease (15).

Summary Fennel and its seeds contain fiber, potassium, magnesium, and calcium — all of which are essential for good heart health.

5. May have cancer-fighting properties

The wide array of powerful plant compounds in fennel may help protect against chronic diseases, including certain cancers.

For example, anethole — one of the main active compounds in fennel seeds — has been found to exhibit cancer-fighting properties.

One test-tube study showed that anethole suppressed cell growth and induced apoptosis, or programmed cell death, in human breast cancer cells (16).

Another test-tube study observed that fennel extract stopped the spread of human breast cancer cells and induced cancer cell death (17).

Animal studies also suggest that extract from the seeds may protect against breast and liver cancer (18).

Although these results are promising, human studies are needed before fennel or its extract can be recommended as an alternative treatment for cancer.

Summary Test-tube and animal studies have shown that fennel may have anticancer properties. However, it’s uncertain whether the same effects would be seen in humans.

6. May benefit breastfeeding women

Fennel has been shown to have galactogenic properties, meaning it helps increase milk secretion. Research suggests that specific substances found in anethole, such as dianethole and photoanethole, are responsible for the galactogenic effects of the plant (6).

Fennel may increase milk secretion and blood levels of prolactin — a hormone that signals the body to produce breast milk (20).

However, other studies found no effect on milk secretion or infant weight gain. Negative side effects, such as poor weight gain and difficulty feeding, have also been reported in infants whose mothers drank lactation teas containing fennel (21, 22, 23).

For these reasons, breastfeeding women should consult their healthcare provider before using fennel to stimulate milk production.

Summary Some studies suggest that fennel may increase milk secretion and weight gain in breastfeeding infants, yet other studies have shown no benefit.

7–10. Other potential benefits

Aside from the benefits mentioned above, fennel and its seeds may improve your health in the following ways:

  1. May have antibacterial properties. Studies show that fennel extract inhibits the growth of potentially harmful bacteria and yeasts, such as Escherichia coli, Staphylococcus aureus, and Candida albicans (24).
  2. May reduce inflammation. The powerful antioxidants in fennel, such as vitamin C and quercetin, can help reduce inflammation and levels of inflammatory markers (25).
  3. May benefit mental health. Animal studies have found that fennel extract may reduce aging-related memory deficits (26).
  4. May relieve menopausal symptoms. A review of 10 studies noted that fennel may improve sexual function and satisfaction in menopausal women, as well as relieve hot flashes, vaginal itching, dryness, pain during sex, and sleep disturbances (27).

It’s important to note that many of these studies used concentrated doses of the plant, and it’s unlikely that eating small amounts of fennel or its seeds would offer the same benefits.

Summary Fennel has antibacterial properties and may improve mental health, relieve menopausal symptoms, and reduce inflammation. Still, it’s unlikely that fennel or its seeds would offer the same effects when eaten in small amounts.


Though fennel and its seeds are likely safe when eaten in moderation, there are some safety concerns over more concentrated sources of fennel, such as extracts and supplements.

For example, fennel has strong estrogenic properties, meaning that it acts similarly to the hormone estrogen. While this may help relieve menopausal symptoms, it may be unsafe for pregnant women.

Due to its estrogen-like activity, there is concern over the plant’s potential teratogenicity — the potential to disturb fetal growth and development.

A study that evaluated the teratogenicity of fennel essential oil showed that high doses may have toxic effects on fetal cells (28).

Although eating fennel and its seeds is likely safe, pregnant women should avoid taking supplements or ingesting the essential oil of this plant.

Fennel may also interact with certain medications, including estrogen pills and certain cancer medications, so always consult your healthcare provider before using high doses in supplement, essential oil, or extract form (29).

Summary Although eating fennel and its seeds is likely safe, consuming higher doses in supplement form may react with certain medications and is unsafe for pregnant women.

The bottom line

Both the flavorful, crunchy bulb and aromatic seeds of the fennel plant are highly nutritious and may offer an abundance of impressive health benefits.

Adding them to your diet may improve heart health, reduce inflammation, suppress appetite, and even provide anticancer effects.

To reap the benefits of fennel and its seeds, try incorporating raw fennel bulb into your salads or using the seeds to flavor soups, broths, baked goods, and fish dishes.


Fennel is an herb also known as Anethum Foeniculum, Anethum piperitum, Bari-Sanuf, Carosella, Fenouil, Finnochio, Foeniculi Antheroleum, Foeniculum, Hinojo, Huile de Fenouil, Phytoestrogen, Sanuf, Shatapuspha, Xiao Hui Xiang, and other names.

Fennel has been used in alternative medicine as a possibly effective aid in treating colic (intestinal gas) in babies who are breast-fed. Fennel may have been combined with other plants or extracts in a specific preparation to treat this condition.

Other uses not proven with research have included indigestion, gas or bloating, bronchitis, cough, or cold symptoms.

It is not certain whether fennel is effective in treating any medical condition. Medicinal use of this product has not been approved by the FDA. Fennel should not be used in place of medication prescribed for you by your doctor.

Fennel is often sold as an herbal supplement. There are no regulated manufacturing standards in place for many herbal compounds and some marketed supplements have been found to be contaminated with toxic metals or other drugs. Herbal/health supplements should be purchased from a reliable source to minimize the risk of contamination.

Fennel may also be used for purposes not listed in this product guide.

Follow all directions on the product label and package. Tell each of your healthcare providers about all your medical conditions, allergies, and all medicines you use.

You should not use this product if you are allergic to fennel or if you have:

  • past or present cancer of the breast, ovary, or uterus; or
  • a history of endometriosis or uterine fibroids.

Ask a doctor, pharmacist, or other healthcare provider if it is safe for you to use this product if you have:

  • allergies to plants such as celery, carrot, or mugwort.

It is not known whether fennel will harm an unborn baby. Do not use this product without medical advice if you are pregnant.

Fennel can make birth control pills less effective. Ask your doctor about using non hormonal birth control (condom, diaphragm with spermicide) to prevent pregnancy while taking fennel.

Fennel may pass into breast milk and may harm a nursing baby. You should not breast-feed while you are taking fennel.

Do not give any herbal/health supplement to a child without medical advice.


Botanical Name of Fennel: Foeniculum vulgare.

Habitat: Fennel is circumpolar (that is, can be found all over the globe). It is most often found in dry stony calcareous soils near the sea.

It is generally considered native to the Mediterranean region but today it is cultivated throughout the British Isles, China, Dominican Republic, Ethiopia, Haiti, Iraq, Italy, Japan, Kurdistan, Malaya, Mexico, Spain, Turkey, and Venezuela.

Plant Description: Fennel is a perennial herb that grows to a height of about 2.5 m.

It has hollow stems, feathery leaves, and flowers that grow in terminal compound umbels about 5-15 cm wide. Each umbel has 20-50 tiny yellow flowers on short pedicels. The leaves are finely dissected, growing up to 40 cm long; the appearance of the leaves is similar to dill leaves but thinner.

The fruit, which is the part most often used in herbal medicine, is a dry seed measuring 4-10 mm long, twice as long as it is wide, with distinct grooves.

Plant Parts Used: Primarily the seeds and an essential oil extracted from them; leaves and root are also used.

The fresh leaves, mostly used as a seasoning and garnish, are not suitable for drying but can be preserved by freezing.

The essential oil is colorless or pale yellow and has a sweet, spicy fragrance reminiscent of anise. The oil is extracted by steam distillation of the crushed seeds.

The Fennel Plant (Foeniculum vulgare) – Attribution: Wouter Hagens

Benefits and Claims of Fennel

The health properties of fennel are warming, carminative, antispasmodic, antidepressant, stomachic, pectoral, diuretic, diaphoretic, aromatic, anti-microbial, pain reducing, fever reducing, and promotes milk-flow in nursing mothers.

Fennel has a long history as a commonly used household remedy for a variety of complaints, especially digestive disorders and it has been used traditionally as a remedy for gas, acid stomach, irritable bowel syndrome (IBS), gout, motion sickness, cramps, and spasms.

Fennel has a diuretic effect, which increases the amount and frequency of urination, which is thought to help remove toxic substances from the body.

Fennel is thought to be an effective herbal remedy for respiratory congestion and is a common ingredient in cough remedies.

It relaxes the smooth muscle lining of the digestive system and is used for cancer patients after radiation and chemotherapy treatments to help rebuild the digestive system.

Fennel is used as a natural treatment for anemia. Fennel contains iron and histidine, an amino acid found in fennel. Histidine stimulates the production of hemoglobin and other components of the blood.

It is common practice, especially in India, to chew fennel seeds after a meal. Some of the components in the plant stimulate secretion of digestive and gastric juices, reduce inflammation of the digestive tract and facilitate proper absorption of nutrients.

Fennel is used as an herbal treatment for diarrhea caused by bacterial action; it is believed that the properties of some components of it are disinfectant and anti-bacterial. Histadine is an example of one of the several amino acids in fennel that aid digestion, helping to relieve diarrhea due to indigestion.

The tea is believed to increase milk production in nursing mothers, and may also relieve colic in babies. An infusion of the bruised seeds is a safe and effective natural treatment for flatulence in babies.

When used with urinary disinfectant herbs like arctostaphylos uva-ursi, the herb is thought to be an effective herbal treatment for cystitis.

The plant is rich in phytoestrogens, and is an emenagogue, easing and regulating menstruation by regulating hormonal action in the body. It is believed to be helpful as a natural treatment for amenhorrea, anxiety and depression.

The Herb Fennel (Foeniculum vulgare)

The ground seed of fennel is made into a tea and taken as a treatment for the symptoms related to snake bites, insect bites or food poisoning. This application is a complementary remedy for symptoms, and must never replace appropriate medical care.

A gargle made from fennel infusion is used as an herbal remedy for relief of hoarseness and sore throats.

The essential oil may be used topically to ease muscular and rheumatic pain and an infusion (steeped tea) of the fennel seeds may be used to prepare a compress as a treatment for conjunctivitis and inflammation of the eyelids.

Besides being used in cooking, fennel is found in many commercial products such as toothpaste, soaps, perfumery, air fresheners, and antacid preparations.

The dried plant is used as an insect repellent; the crushed leaves are used as a flea repellent for dogs and the dried leaves are believed to help keep fleas away from kennels and stables.

Dosage and Administration

Infusion: pour 1 cup boiling water over 1-2 teaspoonfuls of slightly bruised fennel seeds and steep for 10 minutes. Drink up to three times a day. For flatulence, take a cup ½ hour before each meal.

The Fennel Plant (Foeniculum vulgare) – Photo by Carsten Niehaus (Wikipedia)

Side Effects of Fennel

Pregnant women should not take fennel as a medicinal herb internally as it is a uterine stimulant; however, small amounts used in cooking are safe. Skin contact with the sap or essential oil may cause photo-sensitivity or dermatitis in some people.

Ingestion of the essential oil can cause vomiting, seizures and pulmonary edema.

While fennel has a long history of use as a useful herbal remedy, it does contain compounds which are anti-bacterial. Keep in mind that too much of anything can be harmful.

Taken in excess, fennel can induce trouble in breathing, increase palpitations and make heartbeats irregular. Taken in excess, it may also cause neurotic problems.

It is essential to consult a health care professional when altering medications and thoroughly investigate how conventional and herbal medications may interact with each other.

Inform a health care professional of any medications, vitamins or herbs that are currently being used.

It should be kept in mind that herbs may still produce side effects or may carry some element of risk (although side-effects and risks are generally less common and serious than in synthetic drugs).

Any applicable precautions and contraindications should be clearly understood. Carefully research and select a reputable sources when buying herbs, and always read the label for ingredients, instructions and warnings.

Supporting References

Williamson, Elisabeth M.: Potter’s Herbal Cyclopedia. Essex, Saffron Walden 2003.
Balch, Phyllis A.: Prescription for Herbal Healing. New York, Avery 2002.
Tilgner, Sharol: Herbal Medicine. From the Heart of the Earth. Cresswell, Oregon. Wise Acres Press 1999.
Bensky, Dan & Andrew Gamble: Chinese Herbal Medicine. Materia Medica. Seattle, Washington, Eastland Press Inc. 1993.
Bown, Deni: The Royal Horticultural Society New Encyclopedia of Herbs & Their Uses. London, Dorling Kindersley 2002.
Tierra, Lesley: Healing with the Herbs of Life. Berkeley, Crossing Press 2003.
Duke, James A.: The Green Pharmacy Herbal Handbook. Rodale / Reach 2000.
Roybal, Beth Ann Petro & Gayle Skowronki: Sex Herbs. New York, Gramercy Books 2002.
Foster, Steven: 101 medicinal herbs. Loveland, Interweave Press 1998.
Reid, Daniel: A Handbook of Chinese Healing Herbs. Boston, Shambhala 1995.
Hoffmann, David: The New Holistic Herbal. Boston, Element Books Ltd. 1990.
Mills, Simon & Kerry Bone: The Essential Guide to Herbal Safety. St. Louis, Elsevier 2005.

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Thordur Sturluson

Thor Sturluson has a BS in Biology, majoring in Botany, from the University of Maine and a masters degree in Zoology from the Open University in London. He’s an experienced Biologist with a history of working in the environmental services industry. A trained scuba diver and researcher, Thor’s has a keen interest in nature conservation and animal/plant protection. His work and botany passion has made The Herbal Resource what it is.

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Fennel is a spice and a medicinal plant with a long tradition. It may decrease inflammation, relieve menstrual pain, may help with stomach/gut disorders, and may even improve mood, and a number of other disorders and diseases. However, its regular use should be avoided because it can cause some serious side effects. Read on to learn more about health benefits and important safety concerns associated with this plant.

What is Fennel?

Fennel (Foeniculum vulgare) is a plant in the carrot, celery, and parsley family. It is an aromatic herb that has been widely used as a spice and traditional medicine.

It originated in the Mediterranean, but today it is cultivated all over the world .

In Asian cultures, it was ingested to recover from snake bites, as it was believed that it helped eliminate poisons from the body .

Fun facts about fennel:

  • The Greek word “marathon” (for the athletic discipline and the battle the discipline was named after) actually means fennel.
  • Fennel is one of the ancient Saxon peoples’ 9 sacred herbs, credited with powerful healing properties.

Fennel contains many active ingredients, such as :

  • Anethole – this is the main active ingredient of fennel. It is antimicrobial (kills germs), and also mimics estrogen, and increases prolactin.
  • Flavonoids like quercetin and apigenin – these are antioxidant, antibacterial, and anti-inflammatory.
  • Phenolic compounds such as rosmarinic acid and chlorogenic acids – these are antioxidant and anti-inflammatory.
  • Terpenes such as fenchone and limonene, which may improve wound healing.
  • Water-soluble vitamins like ascorbic acid (vitamin C), thiamine (vitamin B1), riboflavin (vitamin b2), niacin (vitamin B3), and pyridoxine (vitamin B6).
  • Fat-soluble vitamins such as vitamins A, E, and K.
  • Trace minerals and other elements like aluminum, barium, calcium, cadmium, cobalt, chromium, copper, iron, magnesium, manganese, nickel, lead, strontium, and zinc.
  • Essential amino acids like leucine, isoleucine, phenylalanine, and tryptophan.
  • Dietary fiber.

Mechanism of Action

Based on animal and cell studies, fennel:

  • Mimics estrogen. Fennel contains compounds such as anethole that mimic estrogen’s function in the body .
  • Increases prolactin (also due to anethole) .
  • Prevents oxidative stress by increasing the content/activities of liver detoxification enzymes (phase I and phase II), and the activity of antioxidant enzymes superoxide dismutase (SOD) and catalase .
  • Decreases inflammation by reducing the production of inflammatory cytokines IL-1beta, IL-6, TNF-alpha, and inflammatory agents NF-κB, MMP2, MMP9, and 5-LOX .
  • Increasing proteins that cause cancer cell death, such as TIMP1, caspase-3, caspase-9, p21, and p27 .
  • Increases levels of acetylcholine (a major neurotransmitter) by blocking acetylcholinesterase .
  • Increases the production of collagen, elastin, TGF-β1, Nrf2, and GSH in the skin .



  • Possibly effective for colic
  • May improve immune stomach/gut discomfort
  • May help with menstrual pain, PMS, and menopause symptoms
  • May help with unwanted hair growth
  • Generally safe with minimal side effects reported
  • A long history of use
  • May improve mood
  • Might reduce appetite
  • May Improve skin health


  • Lack of larger, well-designed clinical trials
  • May cause side effects, including hormonal imbalances, allergies, and seizures
  • May interact with medication

Health Benefits

Possibly Effective For:

“Colic” is a term that refers to when young children cry or are in a state of distress for several hours a day. The causes are unknown, but it is widely believed that stomach cramps and other digestive issues play a key role.

A meta-analysis of 17 studies and a review of 14 clinical trials of supplements for gut disorders concluded that fennel (either as an oil, a tea, or an herbal compound) was effective in treating babies with infantile colic .

In a study with 125 infants, fennel seed oil eliminated colic in 65% of infants, compared to an improvement of only 24% in the placebo group .

2) Improving Stomach/Gut Discomfort

Fennel has a long history of being used to treat a variety of gut and digestive problems, including stomach aches, flatulence, diarrhea, and constipation.

However, at this point in time, there is insufficient evidence to support these benefits.

In a study with 121 people who had Irritable Bowel Syndrome (IBS), a mixture of fennel and curcumin essential oils decreased abdominal pain and improved quality of life .

Anethole, a major component of essential fennel oil, improved stomach function and emptying in rats .

Fennel extract prevented stomach ulcers caused by alcohol in rats .

A small trial that used fennel in a mixture with curcumin and a couple of animal studies cannot be considered sufficient evidence that fennel improves gut discomfort. Larger, well-designed clinical trials are needed to shed light on any potential benefits when it comes to gut health.

3) Alleviating Menstrual Pain

Fennel was found to alleviate menstrual pain in 30 women reporting particularly painful periods (dysmenorrhea). This effect kicked in between 30 – 120 minutes after taking it. However, 5 women withdrew from this study due to finding its odor unpleasant, and one woman reported a mild increase in the amount of menstrual flow .

A combination of fennel extract/vitamin E was more effective than ibuprofen in decreasing the intensity of menstrual pain in a study with 68 female students .

In a study of 80 young women, fennel capsules reduced period-related nausea and improved subjective well-being compared to placebo .

Studies are promising but few and of low quality. Larger trials are needed to establish if fennel is beneficial when it comes to alleviating menstrual pain.

4) Reducing PMS

There is a single study in which this herb reduced the severity of symptoms in 90 young women with moderate to severe PMS .

Other studies are needed to corroborate this effect.

5) Reducing Menopause Symptoms

Menopause is often accompanied by symptoms such as hot flashes, sweating, heart discomfort, sleep problems, depression, irritability, fatigue, anxiety, sexual problems, and joint and muscle discomfort.

In a triple-blind, placebo-controlled trial of 90 women, fennel effectively reduced menopausal symptoms without serious side effects .

However, more studies are needed to confirm fennel is effective in alleviating menopausal symptoms.

6) Unwanted Hair Growth in Women

Sometimes, women with regular menstrual cycles and normal levels of male hormones can experience unwanted hair growth, a condition known as hirsutism (for example, inappropriate facial hair growth).

While the underlying causes are not yet known, fennel has shown potential in reversing this distressing condition.

For example, fennel gels and creams applied directly to the affected areas of the skin reduced hair growth and hair thickness in a DB-RCT of 44 women and in another study of 38 women .

However, larger, well-designed trials are needed to establish that fennel is beneficial for this condition.

7) Anxiety and Depression

In a DB-RCT of 60 post-menopausal women, fennel showed borderline significant improvement in treating anxiety and depression .

Fennel essential oil decreased anxiety in mice .

Further human studies with a larger sample size are required to confirm whether fennel has any actual effect on these conditions.

8) Reducing Appetite

In 9 healthy overweight women, fennel tea decreased hunger and increased the feeling of fullness .

However, this study is small and insufficient to draw any conclusions. Future studies will hopefully clarify if fennel indeed has an effect on appetite.

9) Improving Skin Health

A cream containing fennel extract improved skin texture and increased skin water content in 11 volunteers .

Fennel extract may prevent the visible effects of skin aging that come from sun exposure. For example, it increased the production of collagen, elastin, and TGF-beta1 levels in mice exposed to UVB radiation. Furthermore, it dose-dependently decreased the production of reactive oxygen species (ROS) by increasing Nrf2 and antioxidants such as GSH .

Cell studies have also found that fennel and trans-anethole also lighten the skin and may prevent skin blemishes by blocking UV-induced melanin production (melanogenesis) .

Additional, preferably larger human studies are needed to explore the potential benefits of fennel on skin health.

Lacking Evidence:

No clinical evidence supports the use of fennel for any of the conditions listed in this section. Below is a summary of the existing human, animal, and cell-based research, which should guide further investigational efforts. However, the studies should not be interpreted as supportive of any health benefit.

Breast Milk Production

Fennel has been traditionally used by breastfeeding mothers to increase breast milk production .

However, there are no studies that directly tested the effects of fennel on milk production.

In theory, dopamine blocks prolactin, a hormone that stimulates breast milk production, and anethole, found in fennel, may compete with dopamine at dopamine receptors, thereby blocking the inhibitory action of dopamine on prolactin . This could potentially result in more milk production.

A study of 46 women showed that fennel capsules increased blood prolactin levels .


Historically, fennel has been used as a remedy for many infectious disorders .

Fennel extract prevents the growth of several harmful bacteria in the laboratory (such as staph infection and tuberculosis-causing bacteria, E. coli, and Salmonella) .

It may also prevent the growth of Candida albicans, and harmful molds (Aspergillus niger and Fusarium oxysporum) .


This herb is an abundant source of antioxidants such as caffeoylquinic acid, quercetin, and other phenolic and flavonoid compounds .

Three weeks of fennel supplementation increased the activity of important antioxidant enzymes superoxide dismutase (SOD) and catalase in rats .


Fennel extract has shown significant anti-inflammatory effects in rats and mice. It also prevented slow allergic reactions (type IV allergic hypersensitivity) .

This herb suppressed inflammation in mice with lung injury, by decreasing the production of inflammatory cytokines IL-6 and TNF-alpha and the inflammatory agents MMP9 and nitric oxide (NO) .

Fennel contains several major ingredients that are inhibitors of 5-LOX, an enzyme that causes inflammatory and allergic responses in the body by producing leukotrienes. Fennel’s ability to inhibit these enzymes may make it useful for preventing inflammatory and allergic reactions .

Anethole, found in fennel, decreased inflammatory cytokines IL-1beta and TNF-alpha in a rat model of periodontitis (inflammation of the gums) .


Fennel seeds reduced the growth of skin and stomach tumors in mice. They increased the content/activities of phase I and phase II detox enzymes and the activities of antioxidant enzymes .

Anethole, the principal active component of fennel seeds, increased survival time and reduced tumor weight and volume in mice with cancer .


Fennel extract prevented the deficits in memory in rats and mice caused by aging and scopolamine (a drug that causes amnesia by blocking acetylcholine) .

It blocked acetylcholinesterase, an enzyme that breaks down acetylcholine, in mice . This increases acetylcholine and improves the creation of memories.


Fennel essential oil reduced blood sugar levels in rats with diabetes by nearly 50%. It also prevented diabetes-induced damage to kidneys and pancreas .

In another study, prolonged treatment with fennel distillate improved blood glucose, cholesterol, and glycated hemoglobin in diabetic rats .


In mice with high cholesterol, fennel extract decreased total cholesterol, triglycerides, and LDL-cholesterol (bad cholesterol), while it increased HDL-cholesterol (good cholesterol) .

Another study noticed the increase in HDL-cholesterol in rats .

However, these effects have only been reported in rodents so far, so it’s an open question whether this herb will have as dramatic an effect on cholesterol in human users.

Liver and Kidney Health

Fennel essential oil protected against liver injury and drug-induced liver and kidney damage in rats .

Fennel decreased levels of liver-damage markers aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and bilirubin in both studies .

Blood Pressure

Fennel extract dose-dependently reduced blood pressure in rats .

Blood Clots

Fennel essential oil prevents blood clotting in guinea pig blood and in mice .

Wound Healing

Two key ingredients of fennel, fenchone, and limonene improved wound healing in rats. They act as anti-inflammatory and antimicrobial agents and also increase collagen production .


Fennel, particularly its ingredient anethole, decreased pain in animals without causing sedation. It may work by decreasing inflammatory mediators, which can worsen the sensation of pain .

Airway Relaxation

Fennel extract and essential oil showed an ability to relax the airways (trachea) of guinea pigs .

Dopamine is well known for its role as a neurotransmitter in the brain, but it also plays other roles in the body, such as relaxing the airways to improve airflow to the lungs (bronchodilation) . Anethole, one of the ingredients of fennel, may mimic the effects of dopamine on the respiratory system, thereby achieving a similar effect.


Fennel seed prevented bone tissue from being broken down, leading to improved bone density and bone mineral content in mice .

However, in a DB-RCT in 60 postmenopausal women, short-term (one month) fennel treatment caused no changes in bone density .

Further studies with longer durations are needed to examine the potential benefit of this herb on bone density in humans.

Dosage & Safety


Because fennel is not approved by the FDA for any condition, there is no official dose. Users and supplement manufacturers have established unofficial doses based on trial and error. Discuss with your doctor if fennel may be useful as a complementary approach in your case and which dose you should take.

In most clinical studies, fennel capsules containing 100 mg of active ingredients were given 2 or 3 times per day.

When given in the form of tea, this herb is usually taken half an hour before a meal (to improve digestion), and up to 3 times a day for other purposes.

Fennel seeds are also commonly used as a spice when cooking.


Studies with fennel suggest it is generally safe, with few and minor side effects. It was also safe in infants, although in such cases it was usually only used for relatively short periods of time .

However, although the occasional or short-term use of this herb may have some benefits, regular use should be avoided – mainly because anethole in fennel mimics estrogen, which can cause a hormonal imbalance in the body.

Can Cause Premature Sexual Development

Fennel has sometimes been noted to cause premature thelarche, the early development of breasts, in babies and young children who have not yet begun going through puberty .

However, after cessation of fennel, breast development gradually returns to normal .

May Cause Hyperprolactinemia (High Prolactin)

Fennel increases prolactin levels .

This may be good for some people (such as those looking to stimulate breast milk production), but bad for others who might already be struggling with high prolactin levels.

There are no studies or clinical reports where fennel was directly linked to high prolactin, but the potential for unwanted side effects is still there.

May Cause Galactorrhea

Galactorrhea is the name for the spontaneous or inappropriate flow of milk from the breast, unassociated with childbirth or nursing. It can occur due to high prolactin levels.

There aren’t any studies or reports linking fennel to galactorrhea, but the concern often arises on forums online.

Can Cause Allergies

People who are allergic to plants from the same family (celery, carrot, mugwort) may also be allergic to fennel .

Fennel can cause occupational allergic rhinoconjunctivitis and asthma in people with unusually frequent exposure to this herb, such as cooks and other workers who handle spice on a highly regular basis .

Can Cause Photodermatitis

It contains psoralens, a compound found in many plants and vegetables which can make the skin unusually sensitive to light. This means that large amounts of fennel could make your skin more sensitive to the sun (UV rays), leading to rashes and redness in the skin .

Contains Estragole

Fennel also contains estragole, a compound that has been shown to cause cancer. However, many other herbs and their essential oils also contain this compound .

Research indicates that pure estragole is deactivated by many other substances contained in these plants .

Studies also suggest that estragole may be of little concern when fennel is ingested occasionally and not regularly .

Side-Effects & Precautions

Human studies report few and rare side effects associated with the short-term use of this herb .

  • Nausea (due to the odor)
  • Allergies
  • Mild increase in menstrual flow

Bleeding Disorders (or Being On Blood Thinning Medication)

Fennel may slow blood clotting .

Therefore, it may prolong bleeding and delay wound healing in people with bleeding disorders, or those on blood-thinning medication.


There are a couple of cases where fennel induced seizures in epileptic patients .

Estrogen-Sensitive Disorders and Diseases

Fennel mimics estrogen. If you have any condition that may worsen by exposure to estrogen, do not use it.

Examples of these include breast, uterine or ovarian cancer, endometriosis, and uterine fibroids.

Pregnancy and Breastfeeding

Although fennel is traditionally used in some parts of the world during pregnancy and breastfeeding, there are others who consider it potentially unsafe due to its effects on hormone levels (estrogen and prolactin).

Some people have also claimed that this herb can cause miscarriage, menstrual bleeding, and nerve damage in infants. However, these claims have no scientific backing that we are aware of.

Nonetheless, just as with any other supplement, it’s better to exercise caution and be aware of all possibilities. Always consult your doctor before using any supplements, carefully follow your doctor’s recommendations, and never use fennel instead of what they prescribe.

Drug Interactions

Fennel may block the enzymes CYP2D6 and CYP3A4, which are responsible for breaking down (metabolizing) a number of drugs including opioids, antidepressants, antipsychotics, beta-blockers, antibiotics, and statins. Therefore, taking this herb may enhance the effects of these drugs, potentially causing undesired side effects .

User Reviews

The opinions expressed in this section are solely those of fennel users, who may or may not have medical or scientific training. Their reviews do not represent the opinions of SelfDecode. SelfDecode does not endorse any specific product, service, or treatment.

Do not consider user experiences as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare providers because of something you have read on SelfDecode. We understand that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.

According to users, fennel seeds, capsules, and teas are great for bloating, gas and stomach issues such as IBS. Some have also used it while breastfeeding.

However, frequent use may cause some side effects due to its estrogen-like activity. Some may also experience allergies.

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Fennel is an herb native to southern Europe and Asia Minor. It is cultivated there, and in the US, Great Britain, and temperate areas of Eurasia. All parts of the plant are aromatic. When cultivated, fennel stalks grow to a height of about 1 m. Plants have finely divided leaves composed of many linear or awl-shaped segments. Grayish compound umbels bear small, yellowish flowers. The fruits or seeds are oblong, oval shaped, about 6 mm long, and greenish or yellowish brown in color with 5 prominent ridges. The seeds have a taste similar to anise. Foeniculum dulce (carosella) is grown for its stalks, while F. vulgare (finocchio) is grown for its bulbous stalk bases. A number of subspecies have been identified.

Scientific Name(s)

Foeniculum vulgare

Common Name(s)

Common fennel, sweet fennel, bitter fennel, carosella, Florence fennel, finocchio, garden fennel, large fennel, wild fennel

What is it used for?

Traditional/Ethnobotanical uses

According to Greek legend, man received knowledge from Mount Olympus in the form of a fiery coal held in a stalk of fennel. The herb was known to the ancient Chinese, Indian, Egyptian, and Greek civilizations; the Roman scholar Pliny recommended it for improving eyesight. The name foeniculum is from the Latin word for “fragrant hay.” Fennel was in great demand during the Middle Ages. Wealthy people added the seed to fish and vegetable dishes, while the poor reserved it as an appetite suppressant to be eaten on fasting days. The plant was introduced to North America by Spanish priests and the English brought it to their early settlements in Virginia. All parts of the plant have been used for flavorings, and the stalks have been eaten as a vegetable. The seeds aid digestion. Fennel has been used to flavor candies, liqueurs, medicines, and food, and it is especially favored for pastries, sweet pickles, and fish. The oil can be used to protect stored fruits and vegetables against growth of toxic fungi. Beekeepers have grown it as a honey plant. Health claims have included its use as a purported antidote to poisonous herbs, mushrooms, and snakebites. It also has been used for the treatment of gastrointestinal inflammation, indigestion, to stimulate milk flow in breast-feeding, as an expectorant, and to induce menstruation. Tea made from crushed fennel seeds has been used as an eyewash. Powdered fennel is said to drive fleas away from kennels and stables.

General uses

Fennel has been used as a flavoring agent, a scent, and an insect repellent, as well as an herbal remedy for poisoning and stomach conditions. It has also been used as a stimulant to promote milk flow in breast-feeding and to induce menstruation. However, clinical evidence to support the use of fennel for any indication is lacking.

What is the recommended dosage?

Fennel seed and fennel seed oil have been used as stimulant and carminative agents in doses of 5 to 7 g and 0.1 to 0.6 mL, respectively.


Contraindications have not yet been identified.


Documented adverse reactions and menstruation induction effects. Avoid use.


One study suggested that a fennel constituent has the ability to inhibit the drug metabolizing enzyme cytochrome P450 3A4. Therefore, fennel should be used cautiously with medications metabolized by this enzyme.

Side Effects

Fennel may cause sun poisoning, skin reactions, and cross reactions. The oil may cause hallucinations and seizures. Premature breast development in girls has been reported with the use of fennel. Poison hemlock may be mistaken for fennel.


Fennel oil was found to damage DNA. Estragole, present in the volatile oil, has been shown to cause tumors in animals.

1. Fennel. Review of Natural Products. Facts & Comparisons . St. Louis, MO: Wolters Kluwer Health Inc; May 2012.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer

More about fennel

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Can Eating Too Much Fennel Seed Be Harmful?

  • Carcinogen concern: A November 2018 review of research published in the Annals of Complementary and Alternative Medicine (ACAM) examined a compound found in fennel seeds, known as estragole, as a possible carcinogen. The European Medicine Agency has guidelines on safe amounts of estragole in fennel seeds, given that the natural compound can fluctuate within the fennel plant and its seeds. Most people do not exceed the safe amount of estragole through fennel consumption, according to ACAM. In addition, studies on mice in which liver tumors were caused by pure estragole may not be relevant, because humans appear to metabolize the compound more rapidly. If you use fennel seeds in abundance, asking your doctor about the issue may be relevant, depending on your medical history.
  • Nipple discharge: Women, men and even children can suffer from a condition known as galactorrhea, which causes a milky substance to leak from their nipples. Stimulating milk production is seen as one of many fennel benefits for nursing mothers, which is why it’s included in some lactation tea blends. But a similar hormonal reaction from ingesting large amounts of the seeds can potentially become an unwelcome fennel seed disadvantage.
  • Sun sensitivity: If you take fennel seed regularly, you may be more prone to burning if you don’t wear sunscreen in bright conditions, or if you visit a tanning bed.
  • Drug interactions: Because herbs and spices aren’t regulated by the Food and Drug Administration, there aren’t many official government agency warnings about drug interactions with fennel seeds. The pharmaceutical database Drugs.com suggests that women taking birth control pills use caution, because the seeds may make the pills less effective. Other potential drug interactions include Tamoxifen, a breast cancer treatment drug, and Ciprofloxacin, an antibiotic.

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