Verapamil-rifampin interaction.

A case of verapamil-rifampin interaction is presented in a patient receiving verapamil for supraventricular tachycardia (SVT) and rifampin for pulmonary tuberculosis. The patient experienced recurrent symptomatic SVT, despite receiving verapamil 480 mg po q6h. Serum verapamil concentrations were determined to be extremely low. Discontinuation of rifampin and substitution of ethambutol resulted in an almost four-fold increase in verapamil levels with concurrent control of SVT. Rifampin may have increased the metabolism of verapamil by inducing hepatic microsomal enzymes resulting in low verapamil levels and failure to control SVT.