Taking other medicines Before starting treatment, make sure your doctor knows if you are taking medicines (including those not prescribed by your doctor), such as ketoconazole and itraconazole (used against fungal infections), erythromycin, clarithromycin and rifampicin (used against bacterial infections), phenytoin and phenobarbital (used against seizures), gemfibrozil (used to reduce high levels of fats in the blood such as triglycerides and cholesterol), vitamin A supplements, protease inhibitors (used against viral infections), tamoxifen (used against some forms of cancer) or dexamethasone (used for inflammatory conditions).