Antihistamines are medicines often used to relieve symptoms of allergies, such as hay fever, hives, conjunctivitis and reactions to insect bites or stings.
They're also sometimes used to prevent motion sickness and as a short-term treatment for insomnia.
Most antihistamines can be bought from pharmacies and shops, but some are only available on prescription.
Types of antihistamine
There are many types of antihistamine.
They're usually divided into 2 main groups:
They also come in several different forms – including tablets, capsules, liquids, syrups, creams, lotions, gels, eyedrops and nasal sprays.
Which type is best?
There's not much evidence to suggest any particular antihistamine is better than any other at relieving allergy symptoms.
Some people find certain types work well for them and others do not. You may need to try several types to find one that works for you.
Non-drowsy antihistamines are generally the best option, as they're less likely to make you feel sleepy. But types that make you feel sleepy may be better if your symptoms stop you sleeping.
Ask a pharmacist for advice if you're unsure which medicine to try as not all antihistamines are suitable for everyone.
How to take antihistamines
Take your medicine as advised by the pharmacist or doctor, or as described in the leaflet that comes with it.
Before taking an antihistamine, you should know:
The advice varies depending on the exact medicine you're taking. If you're not sure how to take your medicine, ask a pharmacist.
Side effects of antihistamines
Like all medicines, antihistamines can cause side effects.
Side effects of antihistamines that make you drowsy can include:
Side effects of non-drowsy antihistamines can include:
Check the leaflet that comes with your medicine for a full list of possible side effects and advice about when to get medical help.
If you think your medicine has caused an unwanted side effect, you can report it through the Yellow Card Scheme.
Taking antihistamines with other medicines, food or alcohol
Speak to a pharmacist or GP before taking antihistamines if you're already taking other medicines.
There may be a risk the medicines do not mix, which could stop either from working properly or increase the risk of side effects.
Examples of medicines that could cause problems if taken with antihistamines include some types of:
Try not to drink alcohol while taking an antihistamine, particularly if it's a type that makes you drowsy, as it can increase the chances of it making you feel sleepy.
Food and other drinks do not affect most antihistamines, but check the leaflet that comes with your medicine to make sure.
Who can take antihistamines
Most people can safely take antihistamines.
But speak to a pharmacist or GP for advice if you:
Some antihistamines may not be suitable in these cases. A pharmacist or doctor can recommend one that's best for you.
Always read the leaflet that comes with your medicine to check it's safe for you before taking it or giving it to your child.
How antihistamines work
Antihistamines block the effects of a substance called histamine in your body.
Histamine is normally released when your body detects something harmful, such as an infection. It causes blood vessels to expand and the skin to swell, which helps protect the body.
But in people with allergies, the body mistakes something harmless – such as pollen, animal hair or house dust – for a threat and produces histamine. The histamine causes an allergic reaction with unpleasant symptoms including itchy, watering eyes, a running or blocked nose, sneezing and skin rashes.
Antihistamines help stop this happening if you take them before you come into contact with the substance you're allergic to. Or they can reduce the severity of symptoms if you take them afterwards.