Decreased effects of iron and vitamin B12 in anaemic patients. Phenobarbitone and rifampin reduce efficacy of chloramphenicol. Impairs the action of oral contraceptives. Potentially Fatal: Increases the effect of oral anticoagulants, oral hypoglycaemic agents, phenytoin. Avoid concomitant administration with drugs that depress bone marrow function. May increase serum levels w/ cimetidine and propranolol. Increased risk of cardiac depression w/ ?-blockers and other antiarrhythmics. Additive cardiac effects w/ IV phenytoin. Hypokalaemia caused by acetazolamide, loop diuretics and thiazides may antagonise effect of lidocaine. Dose requirements may be increased w/ long-term use of phenytoin and other enzyme-inducers.