Antihistamine drugs antagonise the action of HISTAMINE and are therefore of value in the treatment of certain allergic conditions (see ALLERGY). They may be divided into those with a central action (e.g. ﬂupheniramine and cyclizine) and those such as loratidine and terfenadine with almost no central action. Antihistamines are also of some value in the treatment of vasomotor RHINITIS (see also under NOSE, DISORDERS OF); they reduce rhinorrhoea and sneezing but are usually less eﬀective in relieving nasal congestion. All antihistamines are useful in the treatment of URTICARIA and certain allergic skin rashes, insect bites and stings, as well as in the treatment of drug allergies. Chlorpheniramine or promethazine injections are useful in the emergency treatment of angio-oedema (see under URTICARIA) and ANAPHYLAXIS.
There is little evidence that any one antihistamine is superior to another, and patients vary considerably in their response to them. The antihistamines diﬀer in their duration of action and in the incidence of side-eﬀects such as drowsiness. Most are short-acting, but some (such as promethazine) work for up to 12 hours. They all cause sedation but promethazine, trimeprazine and dimenhydrinate tend to be more sedating while chlorpheniramine and cyclizine are less so, as are astemizole, oxatomide and terfenadine. Patients should be warned that their ability to drive or operate machinery may be impaired when taking these drugs, and that the eﬀects of ALCOHOL may be increased.