Effect of cimetidine and pyrilamine on histamine-induced ocular surface hyperemia.

Histamine-induced ocular surface redness was virtually abolished by a combination of cimetidine (H2-receptor antagonist) and pyrilamine (H1-receptor antagonist), whereas either antagonist administered alone produced no significant reduction. It appears that combined H1- and H2-antagonist therapy is required for effective blockade of ocular responses to histamine that involve both receptor subtypes.