What does antihistamine do?

What Is an Antihistamine?

By blocking the effects of histamines, this class of drugs can treat allergies and many other ailments.

An antihistamine is a type of medicine used to treat common allergy symptoms, such as sneezing, watery eyes, hives, and a runny nose.

According to the American College of Allergy, Asthma, & Immunology, nasal allergies affect about 50 million people in the United States.

Certain antihistamines are also sometimes used to treat motion sickness, nausea, vomiting, dizziness, cough, sleep problems, anxiety, and Parkinson’s disease.

The drugs work by blocking the effects of histamine, a substance in the body that can cause allergy symptoms.

Antihistamines come in different forms, such as capsules, tablets, liquids, eye drops, injections, and nasal sprays.

They can be purchased over-the-counter (OTC) or given as a prescription.

Some antihistamines are taken daily, while others are used only when symptoms occur.

Types of Antihistamines

Some common antihistamines include:

  • Allegra (fexofenadine)
  • Astelin and Astepro (azelastine) nasal sprays
  • Atarax and Vistaril (hydroxyzine)
  • Benadryl (diphenhydramine)
  • Chlor-Trimeton (chlorpheniramine)
  • Clarinex (desloratadine)
  • Claritin and Alavert (loratadine)
  • Cyproheptadine
  • Dimetane (brompheniramine)
  • Emadine (emedastine) eye drops
  • Livostin (levocabastine) eye drops
  • Optivar (azelastine) eye drops
  • Palgic (carbinoxamine)
  • Xyzal (levocetirizine)
  • Tavist (clemastine)
  • Zyrtec (cetirizine)

Antihistamine Side Effects

Common side effects of antihistamines include:

  • Drowsiness or sleepiness
  • Dizziness
  • Dry mouth, nose, or throat
  • Increased appetite and weight gain
  • Upset stomach
  • Thickening of mucus
  • Changes in vision
  • Feeling nervous, excited, or irritable

Antihistamine Precautions

Before taking an antihistamine, tell your doctor about all medical conditions you have, especially:

  • Diabetes
  • An overactive thyroid (hyperthyroidism)
  • Heart disease
  • High blood pressure
  • Glaucoma
  • Epilepsy (seizure disorder)
  • An enlarged prostate or trouble urinating

Don’t drive or perform activities that require alertness until you know how the antihistamine you’re taking affects you.

Follow the instructions on your prescription or package label carefully when taking an antihistamine. Don’t take more of the medicine than is recommended.

Tell your doctor about all prescription, non-prescription, illegal, recreational, herbal, nutritional, or dietary drugs you’re taking before starting on an antihistamine.

You may need to avoid grapefruit and grapefruit juice while taking an antihistamine, as they can affect how these drugs work in your body. Talk to your doctor if this is a concern.

Antihistamines and Alcohol

Alcohol may worsen certain side effects of antihistamines.

Avoid drinking alcohol while taking these medicines.

Antihistamines and Pregnancy

Tell your doctor if you’re pregnant, or might become pregnant, while using an antihistamine.

You’ll have to discuss the risks and benefits of taking the medicine during pregnancy.

Also, talk to your healthcare provider before taking antihistamines if you’re breastfeeding.

Antihistamines: Understanding Your OTC Options

Path to improved well being

When your body is exposed to allergens (allergy triggers), it makes histamines. Your body releases these chemicals to attack the allergen. Unfortunately, histamines cause the itching, sneezing, runny nose, and watery eyes associated with allergies. Antihistamines treat these symptoms.

First-generation OTC antihistamines

These were among the first antihistamines scientists developed. They are cheaper and widely available. They work in the part of the brain that controls nausea and vomiting. This means they can prevent motion sickness too. The most common side effects of first-generation antihistamines is feeling sleepy. For this reason, they are sometimes used to help people who have trouble sleeping (insomnia).

Some common kinds you can buy over the counter include:

  • Brompheniramine (brand names include Children’s Dimetapp Cold)
  • Chlorpheniramine (brand names include Chlor-Trimeton, Actifed Cold)
  • Dimenhydrinate (brand names include Dramamine)
  • Diphenhydramine (brand names include Benadryl, Nytol, Sominex)
  • Doxylamine (brand names include Vicks NyQuil, Tylenol Cold and Cough Nighttime)

Second-generation OTC antihistamines

These are newer medicines. Many treat allergy symptoms without causing sleepiness. Common kinds include:

  • Loratadine (brand names include Alavert, Claritin)
  • Cetirizine (brand names include Zyrtec)
  • Fexofenadine (brand names include Allegra)

Note: Some antihistamines are mixed with other medicines. These could include pain relievers or decongestants. Many of the brand names above are for these combination medicines. These are meant to treat many symptoms at the same time. It is a good idea to treat just the symptoms that you have. If you have only a runny nose, don’t choose a medicine that also treats headache and fever.

How do I safely take OTC antihistamines?

Read the directions on the label before taking any medicine. Learn how much to take and how often you should take it. If you have any questions about how much medicine to take, call your family doctor or pharmacist. Keep track of which OTC medicines you are using and when you take them. If you need to go to the doctor, take the list with you.

Follow these tips to make sure you are taking the right amount of medicine:

  • Take only the amount recommended on the medicine’s label. Don’t assume that more medicine will work better or quicker. Taking more than the recommended amount can be dangerous.
  • Mixing medicines can be dangerous. If you take a prescription medicine, ask your doctor if it’s okay to also take an OTC antihistamine.
  • Don’t use more than 1 OTC antihistamine at a time unless your doctor says it’s okay. They may have similar active ingredients that add up to be too much medicine.

How can I safely store OTC antihistamines?

Store all medicines out of reach and sight of young children. Store in a cool, dry place so they do not lose effectiveness. Do not store them in bathrooms. These areas can get hot and humid.

Photo: South Agency/Getty Images

Pollen season is in full swing, and I have been sneezing for three weeks straight. My eyes have been itchy for at least two months. When I look back on the last 20 years of my life, I am hard-pressed to think of any substantial period of time in which I was not suffering from one allergy or another. Sure, it’s worst in spring, but then there’s grass to contend with, and later, ragweed, and dust all throughout. The only reprieve comes in the dead of winter, when I might have a cold instead. The other morning, as I thoughtlessly swallowed a Walmart-brand antihistamine from one of several bottles I keep on hand, it occurred to me that I’ve been taking some form of allergy medication nearly every day for as long as I can remember. Is that … bad?

According to David Shih, EVP of strategy and former chief medical officer at CityMD, I am probably fine. Because antihistamines like Claritin and Zyrtec are now available over the counter, we can trust that they’re generally safe for longterm use, says Shih. This is for a couple of reasons. For one, unlike what Shih calls “first generation” allergy medications, like Benadryl, new generation products like Claritin have fewer of the more potentially severe side effects, like sedation. Newer allergy medications are also more effective at targeting the respiratory system, thus leaving the central nervous system and brain alone — for the most part.

Many people do still experience some form of minor side effect with over the counter antihistamines. “The most common side effects you tend to see are fatigue, headaches, and dry mouth,” says Shih. If you’re someone for whom the benefits of regular antihistamine use far outweighs the occasional minor side effect, longterm use is safe for most adults and children, he adds.

All that said: over-the-counter medication is still medication, and should be treated accordingly. “When you’re on these medicines for such a long period of time, sometimes patients tend to forget they’re on it,” says Shih. As a result, many people (me included) fail to list their antihistamine among medications on doctor’s forms, or to tell an urgent-care doctor they’re taking it, and that creates the risk of additive effects when other medications are prescribed. “If you mix with other medication, it can certainly have greater side effects,” says Shih. Alcohol, too, can augment an antihistamine’s side effects.

Antihistamines also fall under a class of drugs known as anticholinergics (a substance that blocks the neurotransmitter acetylcholine in the central and peripheral nervous system), alongside a number of antidepressants as well as certain medications meant to treat Parkinson’s disease and bladder and urinary conditions. Contemporary antihistamines are less likely to produce anticholinergic side effects than older allergy medications (like Benadryl), but it’s not impossible, says Shih — especially if you’re taking more than one anticholinergic medication at a time. If you’re wondering what side effects you should be looking out for, it turns out that there’s a jaunty little saying you can use to remember them. “There’s actually a mnemonic device that we all learned in med school for anticholinergic side effects,” says Shih. Sounding as if he is cursing me, he recites: “Mad as a hatter, dry as a bone, blind as a bat, hot as a hare, red as a beet.” In other words, anticholinergic medications can cause confusion and/or delirium, dry mouth, visual impairment, fever, and flushing. Less fun that it sounds!!

It’s also important to note that not all antihistamines are created equal — while regular Claritin isn’t much different from regular Zyrtec, both come in decongestant forms (Claritin-D and Zyrtec-D), which pose an additional side effect you should know about. “The decongestant can speed up your heart,” says Shih. “It’s important to know if you’re on , and you’re drinking coffee or energy drinks, that can make you jittery and give you an anxiety feeling. That’s not something you take long-term.”

Most people who take generic, non-decongestant antihistamines long-term will be able to stop and start them without issue, but Shih says there are those who do experience some withdrawal. “Some patients who go off these meds may feel generalized itchiness, sneezing, and runny nose.” So, allergies. Cool, great, love it.

7 Unexpected & Potentially Dangerous Side Effects Of Taking Allergy Medicine Long-Term

Sneezing, coughing, and sniffling can be a nuisance, and if you’re someone who suffers from allergies, you might be tempted to take some medication every time a symptom arises. Although some allergy medications are safe to take whenever you need them, there are some side effects of allergy medicine to be aware of if you take it long-term. To make sure you’re not harming your body while trying to get rid of that nasty cough or runny nose, you want to make sure you’re being safe about your medication consumption.

Certain allergy medications work differently depending on the type. “Inhaled steroids and nasal sprays work locally to decrease inflammation in the nasal passages in the lungs, respectively,” allergist and internist Dr. Tania Elliott tells Bustle. “Anti-histamines block the release of histamine from allergy cells. Histamine is one of the main chemicals responsible for causing itch, redness, congestion, and swelling.”

Medications like antihistamines — such as Benadryl or Zyrtec — can be used for long-term treatment. Others such as decongestants or Corticosteriods should not be used for prolonged or too-frequent use. If you are ever in doubt, ask your doctor how long an allergy medication can be used.

“If you don’t want to be on medications longterm, allergy shots or oral allergen immunotherapy may be right for you,” says Elliott. “These treatments can cure your allergies.”

Here are seven unexpected and potentially dangerous side effects of taking allergy medicine long-term, depending on the type of medication, according to experts.

1. Memory Loss

Andrew Zaeh for Bustle

“Some of the older antihistamines cross the blood brain barrier and can cause memory loss and even cognitive impairment,” says Dr. Elliott. These older medications have been linked to dementia, but if you’re sticking to more recent medicines, there isn’t need to worry. Newer generation antihistamines typically don’t have that effect because they don’t work on acetylcholine.

2. Sleep Issues

Andrew Zaeh for Bustle

We all know to avoid taking a Benadryl before getting behind the wheel of the car, and that’s because allergy medicines can cause sleep issues. “Truth is, these medications can impact your daily functioning, so if you are new to them, avoid starting them the night before a big work presentation,” says Dr. Elliott. And if you find they’re causing drowsiness, talk to your doctor about non-drowsy options.

3. High Blood Pressure

Andrew Zaeh for Bustle

Taking decongestants long-term can lead to increased blood pressure and arrythmias. “Any allergy medication with a ‘-D’ at the end of it (Allegra-D, Zyrtec-D) contains a decongestant,” says Dr. Elliott. “They can help in the short-term with drying up your nasal passages and sinuses, but usage beyond seven days can lead to spikes in blood pressure.” Once again, it’s best to talk to your doctor about your symptoms to help mitigate any potential health concerns while taking allergy medications.

4. Vocal Cord Dysfunction

Ashley Batz/Bustle

An inhaler can help you breathe, but sometimes, it can cause issues with your voice. “Some inhaled asthma medications can weaken your vocal cords and cause a hoarse voice, and in some people they can lose their voices almost completely,” says Dr. Elliott. This is not the case for everyone, but speak with your doctor if you start to notice hoarseness as a result of using an inhaler.

5. Diabetes

Ashley Batz/Bustle

Oral steroids such as prednisone are used for severe allergic reactions, eczema, and asthma, and although they help knock out allergy symptoms, they have long-term side effects, including the development of diabetes. “Repeat shorter doses of steroids (for example, a few one week courses a year for asthma or eczema) is enough to start increasing your diabetes risk,” says Dr. Elliott. “Inhaled steroids can do the same thing.” If steroids have been prescribed to you for allergies, talk to your doctor about the risk factor for diabetes to help prevent further health issues.

6. Mood Issues

Ashley Batz/Bustle

Certain long-term asthma medications can cause some emotional disturbances. “In some people, leukotreine modifiers can cause psychological symptoms, such as depression, aggression, irritability, hallucinations and suicidal thinking or behavior,” Sonia Patel, PharmD, Chief Pharmacist at Capsule, tells Bustle. If you begin to develop any of these symptoms, it’s important you talk with your primary physician about how to mitigate these side effects.

7. Osteoporosis

Hannah Burton/Bustle

In addition to increasing your risk of diabetes, long-term steroid use can also affect your likelihood of getting osteoporosis. “Steroids affect the metabolism of both calcium and vitamin D, so long-term use can actually lead to osteoporosis,” says Patel.

If you have persistent allergies or asthma and need to be on medicine long-term, make sure to check with a doctor to see what medication is safest for you.

But this ingredient won’t necessarily knock everyone out. David Rapoport, M.D., director of the Sleep Medicine Research Program and professor of pulmonary and sleep medicine at the Icahn School of Medicine at Mount Sinai, tells SELF that these drugs aren’t very powerful as sleep aids. A 2017 review of the research in the Clinical Practice Guideline for the Pharmacologic Treatment of Chronic Insomnia in Adults examined 46 studies in an attempt to recommend clinical guidelines for treating insomnia. The researchers determined that the evidence behind DPH-based drugs leading to better sleep were “clinically insignificant,” suggesting that people shouldn’t try to use these medications to fall asleep more quickly or stay asleep longer.

However, as Dr. Pelayo points out, how you react to a certain drug is pretty individual, and it’s possible that you might react more strongly due to the placebo effect of taking something you believe will make you sleepy.

Here’s how doctors feel about using DPH to help you fall asleep.

“It’s very attractive to people because it’s an over-the-counter medication,” says Dr. Rapoport. “It’s a quick and dirty way to fix your insomnia.”

But, OK, how bad is it in a pinch? DPH is generally very safe and carries a low risk of serious side effects, so it’s not that big a deal if you turn to it to help you drift off every now and then, Dr. Rapoport says. But there are a couple things doctors want you to be aware of first.

“The big problem with this kind of medication is that it lasts a fairly long time in your system,” Dr. Pelayo says. The lingering effect of a standard adult dose (25 to 50 milligrams, i.e., one to two pills or liquid doses) will likely not be incredibly strong, Dr. Rapoport says, but it may be enough to make you feel sleepy or foggy-headed the next morning. Other side effects are mild and can include dry mouth, nose, and throat; dizziness, constipation; headache; and nausea, according to the National Institutes of Health (NIH).

You can also build up a tolerance to DPH fairly quickly. “ tend to stop working,” Dr. Rapoport says. “The body adapts to them.” With daily use, you may build up a tolerance in about two weeks, he says, though the specific timeline here can vary from person to person. This means you will need to take an increasingly higher dose to achieve the same effect. Higher doses mean a greater risk of side effects, like next-morning sleepiness. If you take a medication with DPH for extended periods, you might get into a vicious cycle of needing more yet feeling even sleepier throughout the day.

Here’s something you may not know: DPH is the sole active ingredient in both Benadryl and ZzzQuil. But only one is indicated for helping you fall asleep.

Although Benadryl lists “marked drowsiness” as a potential side effect, it is not indicated for sleep on its packaging. In fact, Johnson & Johnson, the makers of Benadryl, declined to comment on this subject as it discusses an off-label use.


Antihistamines are mainly used to treat allergies such as hay fever, hives and itching. They may be used to help reduce feeling sick (nausea) and being sick (vomiting).

How do antihistamines work?

Allergic symptoms occur when your body wrongly recognises a food or something in your environment (such as pollen spores) as a threat and sends repair chemicals to deal with these perceived intruders. One of these repair chemicals, histamine, is released from repair cells called mast cells, which are scattered throughout the body.

This histamine can then bind with receptors to trigger increased blood flow to the surrounding area, which can lead to symptoms such as swelling and increased secretions, resulting in a blocked or a runny nose, watery eyes and, most importantly, itchiness.

Antihistamines don’t stop allergic reactions from happening, but they do block the histamine receptors from being able to be triggered by the histamine that is released, reducing your symptoms.

Which antihistamines are available in New Zealand?

Antihistamines come in different forms, including tablets, nasal sprays, eye drops and syrups. There are many brands available on prescription from your GP or over the counter at your local pharmacy. Different antihistamines are better at treating different symptoms, so ask your GP or pharmacist to advise you on which antihistamine is best for your needs.

Generally, antihistamines are classified into 2 main groups – sedating antihistamines and non-sedating antihistamines.

Sedating antihistamines

Sedating antihistamines can make you feel quite drowsy or sleepy. They are used when the effect of drowsiness is helpful to the condition being treated such as in some skin conditions where itch can cause sleep disturbance.

  • Sedating antihistamines may affect your concentration and performance of some tasks that require you to be alert, such as driving, and operating machinery. Take care until you know how these medicines affect you.
  • Limit or avoid alcohol while you are taking sedating antihistamines – alcohol can make the drowsiness worse.

Examples of sedating antihistamines

  • chlorphenamine (Histafen®)
  • dexchlorpheniramine (Polaramine®)
  • doxylamine (Dozile®)
  • promethazine (Phenergan®, Allersoothe®)
  • alimemazine (Vallergan)

Use in children

Sedating antihistamines are often found in cough and cold medicines. These medicines are not recommended for children under 6 years of age. Read more about cough and cold medicines in children.

Sedating antihistamines are not recommended in children under 2 years of age for other conditions.

Non-sedating antihistamines

Non-sedating antihistamines are less likely to cause drowsiness. Although drowsiness is unlikely to happen, it can still occur in some people and may affect the performance of skilled tasks such as driving. Take care until you know how these medicines affect you.

Examples of non-sedating antihistamines

  • cetirizine
  • desloratadine
  • fexofenadine
  • levocetirizine
  • loratadine

How long do antihistamines take to work?

Usually, antihistamine tablets start to work within 30 minutes after being taken and tend to be most effective within 1-2 hours after being taken.

  • Antihistamines are more effective when taken regularly as a prevention, before symptoms occur, rather than only when you have symptoms.
  • This is particularly so for people with hay fever (seasonal allergic rhinitis). In the spring and summer months, the pollen count is generally higher and you may be in contact with the allergen often. Taking the medication regularly will help keep your symptoms under control.

Precautions – before taking antihistamines

Sedating antihistamine, in particular, may not be suitable for some people. If you have any of the following problems, let your doctor or pharmacist know before you start taking any antihistamines:

  • glaucoma
  • problems with your liver or kidneys
  • epilepsy
  • acute porphyria (a rare metabolic disorder)
  • men with prostate enlargement (benign prostatic hyperplasia).

If you are pregnant or breast-feeding check with your doctor before taking antihistamines.

Possible side effects

Most people who take antihistamines do not have any serious side-effects. If side-effects do occur, they are usually minor.

Side effects What should I do?
  • drowsiness
  • dizziness
  • Be careful when driving or using tools until you know how this medicine affects you.
  • These effects put you at risk of falls, and injuries especially if you are elderly. Tell your doctor if you are concerned.
  • Do not drink alcohol.
  • dry mouth, nose or throat
  • headache
  • These are quite common when you first start taking some antihistamines and usually go away with time.
  • Tell your doctor if troublesome.
  • fast or rapid heartbeat
  • unusual weakness
  • difficulty passing urine
  • problems with your eye sight (blurred vision)
  • stomach pain and discomfort
  • Tell your doctor immediately or ring HealthLine 0800 611 116.
  1. Anhistamines New Zealand Formulary
  2. Changes Regarding the Use of Sedating Antihistamines Medsafe Sept 2018

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