Most antihistamines can be bought from pharmacies and shops, but some are only available on prescription.
They also come in several different forms – including tablets, capsules, liquids, syrups, creams, lotions, gels, eye drops and nasal sprays.
Antihistamines work by blocking the action of histamine which the body produces as part of its natural defences.
Histamine is released as a reaction to foreign substances (allergens), and starts a chain reaction that leads to the release of other chemicals that add to the allergic response. There are two different types of antihistamine; sedating antihistamines, which can enter the brain and cause drowsiness and non-sedating antihistamines which do not.
Ask a pharmacist for advice if you're unsure which medicine to try, not all antihistamines are suitable for everyone.
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.
Antihistamines work by stopping a substance called histamine affecting the cells in your body.
Some antihistamines may not be suitable in these cases.
Your pharmacist or doctor can recommend one that's best for you.
Examples of medicines that could cause problems if taken with antihistamines include some types of: It's best to avoid alcohol while taking an antihistamine, particularly if you're taking an older type of antihistamine, as this can increase the chances of it making you feel sleepy.