- Study volunteer who has taken over the counter or prescribed medications, including any enzyme modifying drugs such as Phenytoin, Carbamazepine, barbiturates, St. John wort, glucocorticoids, ketoconazole, itraconazole, erythromycin, clarithromycin and calcium channel blockers or any systemic medication within the past 30 days prior to dosing in first period or any systemic medication within the past 30 days prior to dosing in first period.