Increased clearance w/ enzyme inducers eg phenytoin, barbiturates, primidone, carbamazepine, rifampicin; oxcarbazepine, topiramate, felbamate, griseofulvin, nevirapine, St. John's wort-containing products. Increased plasma levels w/ enzyme inhibitors eg ketoconazole, itraconazole, fluconazole; cimetidine, verapamil; erythromycin, clarithromycin, roxithromycin; diltiazem; ritonavir, saquinavir, indinavir, nelfinavir; nefazodone, fluvoxamine, fluoxetine; grapefruit juice.