Selective inhibitory effects of nifedipine and verapamil on oxidative metabolism: effects on theophylline.

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Abstract
Nine healthy male volunteers were studied to assess the possibility of an interaction between theophylline and nifedipine or verapamil using a randomised, crossover design. Subjects received theophylline 125 mg 8 hourly with and without nifedipine 20 mg 12 hourly and verapamil 80 mg 8 hourly. Nifedipine treatment reduced mean total theophylline clearance by 9%, due to decreased clearances via 1‐ and 3‐ demethylation. Verapamil treatment reduced mean total theophylline clearance by 14%, due to decreased clearances via 1‐ and 3‐ demethylation and 8‐hydroxylation. Verapamil and nifedipine at usual clinical doses are unlikely to cause clinically significant changes in theophylline disposition.