Reported side effects of mexiletine, a useful drug for managing premature ventricular contractions (PVC) and ventricular tachycardia (VT), are gastrointestinal symptoms including nausea and vomiting. Theophylline, a bronchodilator, also causes similar gastrointestinal symptoms with its frequency proportional to the increases in its serum concentration. The occurrence of gastrointestinal symptoms is known to increase in combination therapy of mexiletine and theophylline. However, the pharmacokinetic interaction between mexiletine and theophylline has not been clarified. We, therefore, investigated the effects of mexiletine on theophylline pharmacokinetics. Three patients with bronchial asthma complicating PVC and/or VT (one male and 2 females, aged 70.0 +/- 13.1 years) were studied. All patients were given prophylactic theophylline anticipatory for asthmatic attacks, which was followed by oral administration of mexiletine, 200-300 mg daily, for PVC and/or VT. The serum theophylline concentrations in 3 cases were increased from 13.8 to 25.3 micrograms/ml, from 14.6 to 27.8 micrograms/ml, and from 10.4 to 15.4 micrograms/ml, respectively, after the administration of mexiletine. However, significant decreases in theophylline clearance were observed after the administration of mexiletine by 46, 47 and 35%, respectively. (p < 0.05) With a decrease in theophylline dosage, the serum theophylline concentrations decreased, and gastrointestinal symptoms resolved. Theophylline is metabolized mainly in the liver through an oxidative reaction of p-450 enzyme, however, its metabolism is affected by many factors, such as medications and complications. The results of this study indicated that mexiletine decreases the theophylline clearance by inhibiting the p-450 oxidative reaction to theophylline.(ABSTRACT TRUNCATED AT 250 WORDS)