Antihistamines are drugs that block the effects of histamine. Histamine is a chemical produced by immune cells known as 'Mast Cells'. These cells are involved in killing parasites within our immune systems but in allergic people they also respond to allergens such as pollen and dust mites. By blocking the effects of histamine it is possible to reduce the severity of allergic rhinitis.

If you are taking antihistamines to treat allergic rhinitis due to hayfever then you should usually take them regularly over the pollen season; unless your symptoms are very mild in which case they can be used just for bad days or when the pollen count is high.

In children it is safest to use a non-drowsy anti-histamine such as cetirizine. Some anti-histamines such as chlorphenirimine (Piriton) can cause drowsiness and should be avoided in young children. In addition to oral antihistamines it is also possible to get antihistamine eye drops and nasal sprays. These can be used when symptoms are not severe.