Effect of cimetidine and ranitidine on plasma theophylline in patients with chronic obstructive airways disease treated with theophylline and corticosteroids.

Thirty adults with chronic obstructive airways disease, who were stabilised on theophylline and corticosteroids, took part in a single blind study of the effects of cimetidine and ranitidine on plasma theophylline concentrations. The patients were randomised to receive either 150 mg ranitidine b.d. or 400 mg cimetidine b.d. for one week and serial plasma theophylline measurements were made over a 12-hour period on two consecutive days before, during and after treatment with the H2-antagonist. There was a significant increase in plasma theophylline during treatment with cimetidine; two patients had levels greater than 20 mg.l-1. The average increase in the theophylline concentration due to cimetidine was 32%. There was no significant change in plasma theophylline during ranitidine administration. No adverse effect occurred in any patient during the study.