To the Editor.
—Ranitidine has been reported not to interact with other drugs, including theophylline.1 There are, however, reports of its interaction with metoprolol2 and nifedipine,3 and one report implicating interaction with theophylline.4 We therefore decided to investigate the effect of ranitidine on theophylline.Steady-state basal (12 hours after dose) theophylline levels were measured by fluorescent immunoassay5 (Ames TDA, Ames Division, Miles Laboratories, Elkhart, Ind) on the 1st, 2nd, 5th, and 6th days of ranitidine administration (150 mg twice a day). Ten patients were studied, four females and six males. All patients were in remission and had asthma, except for one male with chronic bronchitis. The mean age of the patients was 32.7±10.7 years (range, 18 to 47 years). None had congestive heart failure, liver disease, or thyroid disease, and none smoked. Patients were receiving long-acting theophylline (200 to 400 mg every 12 hours) therapy