RIFAMPICIN HAS ENZYME INDUCTION PROPERTIES THAT CAN ENHANCE THE METABOLISM OF ENDOGENOUS SUBSTRATES INCLUDING ADRENAL HORMONES, THYROID HORMONES, AND VITAMIN D. RIFAMPICIN CAN ALTER VITAMIN D METABOLISM. IN SOME CASES, REDUCED LEVELS OF CIRCULATING 25-HYDROXY VITAMIN D AND 1,25-DIHYDROXY VITAMIN D HAVE BEEN ACCOMPANIED BY REDUCED SERUM CALCIUM AND PHOSPHATE, AND ELEVATED PARATHYROID HORMONE. IT ACCELERATES THE METABOLISM OF THE FOLLOWING DRUGS:
ANTICONVULSANTS (EG, PHENYTOIN), ANTIARRHYTHMICS (EG, DISOPYRAMIDE, MEXILETINE,
QUINIDINE, TOCAINIDE), ORAL ANTICOAGULANTS, ANTIFUNGALS (EG, FLUCONAZOLE, ITRACONAZOLE, KETOCONAZOLE), BARBITURATES, BETA-BLOCKERS, CALCIUM CHANNEL BLOCKERS (EG, DILTIAZEM, NIFEDIPINE, VERAPAMIL), CHLORAMPHENICOL,
CORTICOSTEROIDS, CYCLOSPORINE, CARDIAC GLYCOSIDE PREPARATIONS, CLOFIBRATE, ORAL
OR OTHER SYSTEMIC HORMONE CONTRACEPTIVES, DAPSONE, DIAZEPAM, DOXYCYCLINE,
FLUOROQUINOLONES (EG CIPROFLOXACIN), HALOPERIDOL, ORAL HYPOGLYCEMIC AGENTS
(SULFONYLUREAS), LEVOTHYROXINE, METHADONE, NARCOTIC ANALGESICS, NORTRIPTYLINE,
PROGESTINS, TACROLIMUS, THEOPHYLLINE AND ZIDOVUDINE. IT MAY BE NECESSARY TO ADJUST THE DOSAGES OF THESE DRUGS IF THEY ARE GIVEN CONCURRENTLY WITH RIFAMPICIN. PATIENTS USING ORAL OR OTHER SYSTEMIC HORMONAL CONTRACEPTIVES SHOULD BE ADVISED TO CHANGE TO NONHORMONAL METHODS OF BIRTH CONTROL DURING RIFAMPICIN THERAPY.
RIFAMPICIN HAS BEEN OBSERVED TO INCREASE THE REQUIREMENTS FOR ANTICOAGULANT DRUGS
OF THE COUMARIN TYPE. IN PATIENTS RECEIVING ANITCOAGULANTS AND RIFAMPICIN CONCURRENTLY, IT IS RECOMMENDED THAT THE PROTHROMBIN TIME BE PERFORMED DAILY OR AS FREQUENTLY AS NECESSARY TO ESTABLISH AND MAINTAIN THE REQUIRED DOSE OF ANTICOAGULANT. DIABETES MAY BECOME MORE DIFFICULT TO CONTROL. CONCURRENT USE OF KETOCONAZOLE AND RIFAMPICIN HAS RESULTED IN DECREASED SERUM CONCENTRATIONS OF BOTH DRUGS. CONCURRENT USE OF RIFAMPICIN AND ENALAPRIL HAS RESULTED IN DECREASED CONCENTRATIONS OF ENALAPRILAT, THE ACTIVE METABOLITE OF ENALAPRIL. CONCOMITANT ANTACID ADMINISTRATION MAY REDUCE THE ABSORPTION OF RIFAMPICIN. DAILY DOSES OF RIFAMPICIN SHOULD BE GIVEN AT LEAST 1 HOUR BEFORE THE INGESTION OF ANTACIDS. PROBENECID AND COTRIMOXAZOLE HAVE BEEN REPORTED TO INCREASE THE BLOOD LEVEL OF RIFAMPICIN. WHEN RIFAMPICIN IS GIVEN CONCOMITANTLY WITH EITHER HALOTHANE OR ISONIAZID, THE POTENTIAL FOR HEPATOTOXICITY IS INCREASED. PLASMA CONCENTRATIONS OF SULFAPYRIDINE MAY BE REDUCED FOLLOWING THE CONCOMITANT ADMINISTRATION OF SULFASALAZINE AND RIFAMPICIN. THIS FINDING MAY BE THE RESULT OF ALTERATION IN THE COLONIC BACTERIA RESPONSIBLE FOR THE REDUCTION OF SULFASALAZINE TO SULFAPYRIDINE AND MESALAMINE.