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 which have a sedative effect (e.g. chlorpheniramine, diphenhydramine) and those such as loratidine and cetirizine, which have little or no such 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, although they vary in the degree of sedation they might cause and in their duration of action. Patients are normally 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.