1 There is evidence that H2 receptors are present in the lung, both on bronchial smooth muscle and mast cells. In animal studies, stimulation of H2 receptors causes a diminution, and conversely H2 receptor blockade can increase, smooth muscle contraction and mediator release. 2 The effects of H2 receptor blockade in ten patients with asthma has been studied using oral cimetidine in a dose of lg daily for 1 week. Treatment was compared with placebo and the H1 receptor antagonist chlorpheniramine. 3 There was no alteration in the severity of naturally-occurring or exercise-induced asthma with cimetidine or chlorpheniramine. 4 H2 receptor blockade with oral cimetidine in conventional doses is without ill effect in asthma. The use of larger parenteral doses is discussed.

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