Enhanced effects of antihypertensive agents, nitrates, dihydropyridine Ca antagonists, theophylline, triazolam, midazolam, muscle relaxants. Decreased effect w/ rifampicin. Increased adverse reactions w/ β-blockers, Rauwolfia prep, digitalis, antiarrhythmic drugs, aprindin HCl, cyclosporin, tacrolimus, carbamazepine, phenytoin, cimetidine, HIV protease inhibitors, anesth.