The influence of antituberculosis drugs on the plasma level of verapamil.

The influence of antituberculosis drugs on the plasma level of verapamil was studied after its oral and intravenous administration. Six patients who had been treated for at least 6 months with a combination of rifampicin, ethambutol and isoniazid received a single oral dose of 40 mg verapamil. As compared to untreated subjects, the antituberculosis drugs greatly reduced the bioavailability of the calcium antagonist. Studies in patients in whom treatment with ethambutol and isoniazid had been discontinued revealed that the effect was due to rifampicin. The drugs for tuberculosis had no influence on the plasma level of verapamil when it was given intravenously. The findings can be explained by the induction of verapamil metabolizing liver enzymes in patients treated with rifampicin.