- Taking any of the following drugs at the time of study participation: epidermal growth factor receptor inhibitor, topoisomerase 1 inhibitor (camptothecin, irinotecan); buspirone, benzodiazepines, zolpidem, calcium channel blockers (such as felodipine, nifedipine, verapamil); digoxin or quinidine; codeine or fentanyl; phenytoin, propranolol, rifampin, or theophylline