Increased clearance w/ phenytoin, phenobarb, primidone, bosentan, carbamazepine, rifampicin, some HIV PIs (eg, ritonavir) & NNRTIs (eg, efavirenz, nevirapine), & possibly oxcarbazepine, topiramate, rifabutin, felbamate, griseofulvin, St. John's wort. Increased/decreased plasma conc w/ combinations of HIV PIs (eg, nelfinavir) & NNRTIs (eg, nevirapine), &/or combinations of HCV medicinal products (eg, boceprevir, telaprevir). Increased serum conc w/ strong (eg, ketoconazole, itraconazole, clarithromycin) or moderate (eg, fluconazole, diltiazem, erythromycin) CYP3A4 inhibitors. Increased plasma conc of ethinylestradiol w/ etoricoxib. Increased plasma & tissue conc of ciclosporine. Decreased plasma & tissue conc of lamotrigine. Weak increases in plasma conc of theophylline. Moderate increases in plasma conc of tizanidine.