Antihistamines are medicines that are often used to relieve the symptoms of hay fever (allergic rhinitis) and some other allergies such as urticaria (hives). They work by blocking the action of histamine, a chemical messenger our bodies produce in response to what it sees as a threat, producing immediate responses such as sneezing, runny nose and eye watering.Antihistamines fall into two groups:Older, first-generation ones which tend to make you drowsy. Some people may prefer these when their hay fever is affecting their sleep.Newer-generation antihistamines which are less likely to make you drowsy, which makes them popular for relieving symptoms throughout the day.ChlorphenaminePromethazineHydroxyzineCetirizineLoratadineAcrivastineFexofenadine (prescription only)Most of these are available at pharmacies or at supermarkets, except for fexofenadine which is prescription-only. They are usually available in tablets or liquid form, and some are available as eye drops too. People often find that one particular antihistamine works better for them. Seek help from your pharmacist to help you find your choice of antihistamine.