Actions of terfenadine and cimetidine on histamine wheal formation.

1. A Latin square design was used to compare the effects of four masked oral drugs, placebo, cimetidine, terfenadine and cimetidine plus terfenadine, at single standard doses against histamine injected at concentrations of 0.3 mM and 0.074 mM subepidermally in eight healthy volunteers studied at weekly intervals. 2. Wheal growth occurred in three phases, an initial delay, a phase of rapid growth where drugs were effective, and a phase of slow growth where there were no significant drug effects. 3. Terfenadine was an effective antihistamine. 4. The data were consistent with a terfenadine effect continued into the next treatment block. 5. Under these conditions cimetidine apparently reduced the effects of terfenadine during the rapid wheal growth phase both simultaneously and after a week's interval from terfenadine dosing.