Expert reviewer, Alastair Bovell, Lead Clinical Pharmacist
Next review due March 2020

Antihistamines are medicines commonly used to stop the symptoms of an allergic reaction.

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Why has my doctor advised me to take antihistamines?

Your doctor may suggest taking antihistamines to relieve the symptoms of a common allergy you’re having. You can also buy some antihistamine medicines over the counter.

For example, you may get an allergic reaction to:

  • pollen (hay fever)
  • dust
  • certain food
  • insect bites or stings
  • animal hair and dead skin cells
  • latex
  • medicines
  • mould

This may trigger symptoms such as:

  • sneezing
  • runny nose
  • wheezing
  • skin rash
  • itching
  • sore eyes
  • swelling

Antihistamines can help ease these symptoms.

Because they help dampen the immune response (see How do antihistamines work, below), they can be used to help treat skin conditions such as hives (urticaria), eczema and dermatitis. These conditions happen when your immune system reacts inappropriately to a particular trigger (allergen) – this is known as an allergic reaction.

Conjunctivitis can sometimes be caused by an allergic reaction, so antihistamines may help.  

Antihistamines have other uses too. They can help ease nausea (including in travel sickness) and vertigo, relieve asthma and aid sleep. Antihistamines have muscle-tightening properties that can prevent incontinence and manage conditions like irritable bowel syndrome. They are also added to medicines to treat coughs and colds.

How do antihistamines work?

Your immune system naturally releases a chemical called histamine to protect you from pathogens, such as viruses and bacteria, that can cause illness and harmful conditions.

But if you have an allergy to something that isn’t actually harmful to you, such as pollen, your body has an inappropriate reaction to it and releases too many histamines.

These may then trigger irritation in your:

  • lungs
  • throat
  • nose
  • sinuses
  • ears
  • stomach lining
  • skin

Antihistamines work by blocking the effect of histamine in your body. This helps to prevent inflammation and eases allergic reactions.

If you have a severe allergic reaction, your symptoms may include:

  • difficulty breathing
  • swollen lips and eyelids
  • a fast heartbeat
  • vomiting

This type of severe reaction is called anaphylaxis. You may be at particular risk if you have asthma. Anaphylaxis is commonly triggered by an allergy to wasp and bee stings, latex, and certain foods, including eggs, fish, shellfish, cow’s milk and nuts. Some medicines may also cause anaphylaxis.

If you have anaphylaxis, it’s important that you get medical help immediately, as it can be very serious and you need an adrenaline injection. If you have an adrenaline device – such as an Epipen® – you or a trained individual need to inject it and seek immediate medical help. If you don’t have a device yourself, you may be given the adrenaline by a paramedic or hospital staff. You may then need further treatment, for example with steroids and antihistamines, to reduce the allergic response.

What are the different types of antihistamines?

Antihistamines come in different forms. For example, there are tablets, syrups, sprays and drops. You can get antihistamines over the counter from your pharmacy and certain local high-street stores, but some will need to be prescribed by your doctor. There are currently two main types of antihistamine: sedating and non-sedating.

Sedating antihistamines

These are the older or ‘first generation’ antihistamines. They have a more sedative effect because the medication moves from your bloodstream to your brain. This can make you drowsy and slow down your thinking and movement.

Your doctor may prescribe sedating antihistamines if you have problems sleeping, especially if severe itching from a skin condition keeps you awake.

Sedating antihistamines that you can buy include the following.

Medicine: chlorphenamine maleate
Brand name: Piriton®

Medicine: clemastine
Brand name: Tavegil®

Medicine: cyproheptadine hydrochloride
Brand name: Periactin®

Medicine: promethazine hydrochloride
Brand name: Phenergan®, Night Nurse®

Non-sedating antihistamines

Newer ‘second generation’ types of antihistamine are often called non-sedating antihistamines. They have less effect on your brain and won’t make you so drowsy, though it varies from person to person. They usually have fewer side-effects too.

Non-sedating antihistamines that you can buy include the following.

Medicine: acrivastine
Brand name: Benadryl Allergy Relief®

Medicine: cetirizine hydrochloride
Brand name: Piriteze Allergy Relief®, Pollenshield Hayfever Relief®, Benadryl One A Day Relief®, Zirtek Allergy®

Medicine: loratadine
Brand name: Clarityn®

Some antihistamines (both sedating and non-sedating) are also available as pharmacy own-brands.

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How do I take antihistamines?

There are various ways to take antihistamines, including:

  • nasal sprays
  • eye drops
  • tablets and capsules
  • liquids and syrups (especially for young children)

You can buy some of these from pharmacies without a prescription. These include well-known hay fever and allergy products like Piriteze® (cetirizine) and Benadryl® (cetirizine or acrivastine) and pharmacy own-brands. You can also buy some antihistamines to help with bites and stings, travel sickness and helping you sleep. Commonly used medicines to treat colds often combine antihistamines with decongestants and painkillers.

Other antihistamines are only available on prescription because they may have side-effects or interact with other medicines you might be taking.

Your doctor may recommend antihistamines that you don’t need to take so often. This can make it easier to stick to the recommended dose.

Be careful not to overuse antihistamine eye drops. This can make some people more susceptible to glaucoma.

For information about taking antihistamines, see our FAQ: Best method.

Monitoring your health

As well as taking antihistamines to treat the symptoms, it’s important to manage whatever is causing your symptoms. If you’re allergic to something, you should try to avoid it. A food diary might be helpful here. You record what you have eaten so you can see if there is a pattern or a particular food that might be causing the allergic reaction. A similar diary might be a good idea if you used a different washing powder, soap or detergent around the time your symptoms started.

Depending on how severe your reaction and symptoms are, your GP may suggest you see an allergist or immunologist. These are health professionals who specialise in diagnosing allergies and finding ways to stop them getting worse.

They may suggest a ‘skin prick test’ to check exactly what triggers your allergic reaction. You need to stop taking antihistamines (including some cough and cold medicines) several days before the test.

Can anyone take antihistamines?

Antihistamines are not suitable for everyone. If any of the information below applies to you, it’s important that you speak to your pharmacist or GP before taking them.

If you’re pregnant or breastfeeding

If you’re pregnant, it’s generally recommended that you don’t take antihistamines. They can affect your baby, especially if you take them in the later stages of pregnancy. This is also true if you’re breastfeeding because antihistamines can get into your breast milk.

Ask your GP for advice if you feel you need to be treated with antihistamines while pregnant or breastfeeding. For example, severe itching may be affecting your sleep and overall health. The non-sedating antihistamine loratadine is usually seen as safe, and the sedating type chlorphenamine may also be suitable.

If you have a health condition

If you’ve got epilepsy, get advice from your GP before taking a sedating antihistamine. You shouldn’t take this type of antihistamine if you have severe liver disease although some non-sedating versions are safe.

The muscle-tightening effects of some antihistamines mean they’re unsuitable if you already retain too much urine (that is, you have urine retention).

If you have heart problems

A particular type of antihistamine called hydroxyzine has been found to affect how your heart beats. If you have certain heart problems, your doctor will only prescribe this at the lowest possible dose for the shortest possible time.

For children and older people

Most antihistamines are suitable for children and older people, though they’re more likely to get side-effects.

If you wear contact lenses

Soft contact lens wearers should take their lenses out before starting treatment with antihistamine eye drops. These drops contain a preservative that builds up in soft lenses and will irritate your eyes. It also causes clouding in the lenses so you can’t see so well.

Interaction of antihistamines with other medicines

Check with your pharmacist or GP to find out if taking an antihistamine will interact with medicines you’re already taking.

Tricyclic antidepressants interact with antihistamines and can increase the drowsiness side-effect.

Antibiotics and medicines for fungal infections can make any non-sedating antihistamines in your body more concentrated.

Some antihistamines incorporate other medicines for the best effect. For instance, azelastine nasal spray also contains corticosteroids to stop the sneezing and runny nose you often get with hay fever.

It’s best not to drink alcohol while taking any type of antihistamine. It increases drowsiness and slows your thinking and movement.

Side effects of antihistamines

This section doesn’t include every possible side-effect of antihistamines. Always read the patient leaflet that comes with your medicine and ask your pharmacist for advice.

The sedating antihistamines will make you feel very drowsy and can affect your coordination, movement, reactions and thinking. You may find they affect your learning and academic performance.

These effects can take time to wear off. So you shouldn’t drive, operate machinery or do other complex tasks for 24 hours after taking a sedating antihistamine.

With the newer, non-sedating antihistamines, drowsiness is less of a problem, but they may still slow down your movement and thinking.

Alcohol can increase the sedative effect of both types of antihistamine, so you should avoid it.

If eye drops for conjunctivitis affect your vision, wait till the effects wear off before driving or operating machinery.

Common side-effects of antihistamines, especially sedating types, include:

  • headache
  • difficulty passing urine
  • dry mouth
  • blurred vision
  • other eye problems, such as dryness and stinging
  • feeling sick or vomiting

Other, rare side-effects of antihistamines include:

  • low blood pressure
  • arrhythmia (abnormal heart rhythm)
  • dizziness
  • confusion
  • depression
  • disturbed sleep
  • spasms, jerkiness and tremor (a slight shaking) in parts of your body, often your hands
  • allergic reactions (including swelling, rashes, breathing difficulties)
  • blood or liver problems

Older people (adults over 65) are particularly likely to become confused and at risk of having a fall when taking antihistamines. Sedating antihistamines can actually have a stimulating effect on older people and also make children hyperactive.

If you use an antihistamine nasal spray, you may find it leaves a bitter taste. This may be because you’re not using it correctly.

Frequently asked questions

  • Antihistamines are generally very safe but it is possible to overdose on them. Many different medicines contain antihistamine; examples include medicines for coughs and colds. You could easily reach the maximum safe dose if you take an antihistamine at the same time as taking one of these medicines. Ask your pharmacist for advice.

    If you take too much of a sedating antihistamine, your speech, and movement may become uncoordinated (ataxia). Your mouth might get dry and it will be hard to swallow. Your blood pressure may go up or down. You may feel agitated and detached from reality. An overdose can lead to fits, but this is uncommon.

    One reason you may get drowsy with a non-sedating antihistamine is because you’ve had more than the recommended dose. With this type of antihistamine, you may also get headaches and a stomach upset if you take too much.

    An overdose of either type of antihistamine can make your heart beat faster (tachycardia) or in an abnormal rhythm (arrhythmia).

    If you think you’ve taken too much of any antihistamine, you should get medical help straight away. The effects of an overdose will usually appear within six hours.

    If you feel unwell, go to your nearest accident and emergency department to have your symptoms checked. Take the medicine packet(s) with you, so the doctor can see exactly what type of antihistamine you have taken.

    Your specialist (dermatologist or immunologist) may prescribe a higher dose of antihistamine than is licensed (more than is suggested on the packet or patient information leaflet). Only a specialist will do this, and it will depend on why you need it.

  • It depends on what your main symptoms are, how fast they come on and how often you have them. There may be other factors like convenience and how quickly you want the medicine to work.

    For instance, in the case of an insect sting or bite, you will want to treat it immediately. For this reason it is a good idea to keep suitable medication to hand, perhaps in a first aid kit. But if you know you’re going somewhere there are animals, you might want to take something in advance to prevent an allergic reaction.

    If you get summer hay fever, it’s best to start taking antihistamines a couple of weeks before the season begins. Different pollens have different seasons so it may not just be the summertime when you take them. All-year-round hay fever may mean you need to use antihistamines regularly for a long time.

    Your pharmacist can explain the different properties of antihistamine products available and show you how to use the various methods, like nasal sprays and eye drops.

    Some forms of antihistamine act quickly and start working in 15 to 20 minutes. Others may take more time to work but have a longer-lasting effect on your symptoms.

    Tablets are easier to carry with you, but you may need water to take them, unlike syrups or liquids. You can now also get ‘fast-melt’ antihistamines that dissolve in your mouth.

    If you’re allergic to milk, you might want to use syrups because tablets contain lactose. The tablets will be free from cow’s milk but the lactose may still cause a reaction in some people. Syrups may not be a good idea if you’re diabetic or intolerant to sugar.

    Children may prefer a sweetened syrup, and it can be easier to pour out a precise dose for them to take.

    Talk to your pharmacist or GP if you're unsure which treatment is best for you or if what you’re using isn’t controlling your symptoms very well. Tell them how easy or difficult you find it to take medicines regularly, or several times a day. Some antihistamines are only recommended for use for a short period.

    If you think a medicine is not working well for you, it might be that you’re not taking it in the most effective way. For instance, if you’re using an antihistamine nasal spray, it’s possible that you’re not applying it correctly. Your pharmacist or GP will be able to help.

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  • Reviewed by Natalie Heaton, Specialist Health Editor, Bupa Heath Content Team, March 2017
    Expert reviewer, Alastair Bovell, Lead Clinical Pharmacist
    Next review due March 2020

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