May enhance effects of L-dopa, dopaminergic agonists, & anticholinergics. May reduce effects of barbiturates & neuroleptics. May modify effects of antispasmodic agents, dantrolene or baclofen. Risk of pharmacotoxic psychosis w/ amantadine & potentially ketamine, dextromethorphan & phenytoin. Potential risk of increased plasma levels w/ other active substances that use the same renal cationic transport system eg, cimetidine, ranitidine, procainamide, quinidine, quinine & nicotine. Possible reduced serum level of hydrochlorothiazide. Increased INR w/ warfarin.