Loratidine Health Dictionary

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. ?upheniramine 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 e?ective 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 di?er 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.... antihistamine drugs

n. a drug that inhibits the action of *histamine by blocking specific histamine receptors. Four histamine receptors have been identified (H1 to H4). H1-receptor antagonists are used for symptomatic relief of allergic conditions, such as hay fever, pruritus (itching), and urticaria (nettle rash). Many H1-receptor antagonists, e.g. *cyclizine and *promethazine, also have a strong *antiemetic activity and are used to prevent motion sickness. The most common side-effect of these drugs, especially the older antihistamines (e.g. *alimemazine, promethazine), is drowsiness and because of this they are sometimes used for sedation. Newer antihistamines, e.g. cetirizine, loratidine, mizolastine, are less sedating. Other side-effects include dizziness, blurred vision, tremors, digestive upsets, and lack of muscular coordination.

H2 receptors are mainly found in the stomach, where stimulation by histamine causes secretion of acid gastric juice. H2-receptor antagonists (e.g. *cimetidine, *nizatidine, *ranitidine, and *famotidine) block these receptors and reduce gastric acid secretion; they are used in the treatment of functional dyspepsia, *peptic ulcers, and *gastro-oesophageal reflux disease. H3- and H4-receptor antagonists have yet to find a clinical role.... antihistamine