Antihistamines: the old and the new.

Several pharmacologic agents provide antihistamine effects by acting at the H1 histamine receptor site. The classic agents are relatively nonselective, resulting in a wide range of effects, both therapeutic and undesirable. The newer agents preferentially block peripheral H1 receptor sites and, consequently, have fewer side effects, including sedation. Antihistamines are useful in the treatment of allergic conditions, Parkinson's disease, insomnia and some forms of nausea, and provide symptomatic relief of cough and other conditions associated with respiratory tract infections. Certain agents may play a role in the treatment of asthma and anorexia. Selection of a specific agent should be based on cost and the minimization of side effects. The classic antihistamines provide an inexpensive and highly effective means of treating histamine-mediated symptoms. The bothersome central nervous system side effects can be alleviated by taking the drugs at bedtime; their prolonged tissue half-life allows dosing once or twice a day for 24-hour clinical relief. The newer, more expensive nonsedating antihistamines are acceptable alternatives for patients who are incapable of tolerating the effects of classic agents.