Additive antimuscarinic action of atropine, TCAs, MAOIs. Increased effects of antidepressants, alcohol, antihistamines, sedatives, pain relievers, anxiety medicines, muscle relaxants. Additive CNS depression w/ barbiturates, tranquilizers, alcohol. Reduced l-dopa effects. Reduced altretamine activity. Decreased phenobarb & phenytoin serum conc. Increased pyridoxine requirements w/ hydralazine, INH, penicillamine, OCs.