Abstract: The potential for continuous infusion of cimetidine to affect the clearance of aminophylline was assessed in 18 critically ill patients treated for bronchospasm. This was a pharmacokinetic drug‐drug interaction study in which the subjects were administered aminophylline as a continuous infusion at low doses (mean 10.8 mg/hr). Subjects were started on cimetidine (50 mg/hr) 24 hours post aminophylline drip and remained on it for 48 hours. Theophylline clearance was determined right before and 48 hours after starting cimetidine. There were no significant differences in theophylline clearance before and after infusion of cimetidine (P > .05). Based on our findings, cimetidine does not seem to affect the clearance of low doses of theophylline in critically ill patients.