Decreased hepatic clearance w/ cimetidine, allopurinol (high-dose), propranolol, macrolides (eg, erythromycin, clarithromycin), quinolones (eg, ciprofloxacin, enoxacin), alcohol, OCs, mexiletine, tacrine, thiabendazole, disulfiram, interferon-α & verapamil. Increased hepatic clearance w/ tobacco or marijuana smoking, phenobarbitone, phenytoin, carbamazepine & rifampicin. Reduced plasma levels w/ St. John's wort. Ventricular arrhythmias w/ halothane. May lower seizure threshold w/ ketamine. Enhanced renal clearance of lithium. Synergism w/ adrenaline & other sympathomimetic amines. Cardiac arrhythmias w/ β-adrenergic agonist. Ranitidine, diltiazem, nifedipine, INH, frusemide, flu vaccine & corticosteroids.