See Contraindications. Reduces serum levels of protease inhibitors: avoid concomitant use. Antagonizes anticonvulsants, digitoxin, antiarrhythmics, oral anticoagulants, antifungals, barbiturates, β-blockers, calcium channel blockers, chloramphenicol, clarithromycin, corticosteroids, cyclosporine, cardiac glycosides, clofibrate, oral or other hormonal contraceptives (consider nonhormonal methods), dapsone, diazepam, doxycycline, fluoroquinolones, haloperidol, oral hypoglycemics, levothyroxine, methadone, narcotic analgesics, progestins, quinine, tacrolimus, theophylline, tricyclic antidepressants, zidovudine, atovaquone, enalapril, sulfasalazine. May be potentiated by atovaquone, probenecid, cotrimoxazole. Concomitant ketoconazole decreases serum concentration of both drugs. Monitor prothrombin time with concomitant anticoagulants. Separate dosing of antacids by at least 1 hour. Avoid concomitant halothane. May cause false (+) urine tests for opiates. May inhibit assays for serum folate and Vit. B 12 (consider alternative methods). Perform LFTs and use contrast media prior to morning dose.