The R-enantiomer of the calcium channel blocker verapamil. Dexverapamil competitively inhibits the multidrug resistance efflux pump P-glycoprotein (MDR-1), thereby potentially increasing the effectiveness of a wide range of antineoplastic drugs which are inactivated by MDR-1 mechanisms. This agent exhibits decreased calcium antagonistic activity and toxicity compared to racemic verapamil.