CAUSES DECREASED EFFICACY OF OTHER DRUGS LIKE ORAL ANTICOAGULANTS, ORAL CONTRACEPTIVES, PHENYTOIN.VIT B6 DECREASES ITS SERUM CONC. SEDATION OR RESPIRATORY DEPRESSION MAY BE ENHANCED BY DRUGS WITH CNS-DEPRESSANT PROPERTIES; IN PARTICULAR ALCOHOL. BARBITURATES GENERALLY INDUCE LIVER ENZYMES. MAOLS MAY PROLONG THE CNS DEPRESSANT EFFECTS OF SOME BARBITURATES, PROBABLY BY INHIBITION OF THEIR METABOLISM. HOWEVER. MAOLS LIKE OTHER ANTIDEPRESSANTS ALSO REDUCE THE CONVULSIVE THRESHOLD AND THEREBY ANTAGONISE THE ANTICONVULSANT ACTION OF BARBITURATES. ANALGESICS - DEXTROPROPOXYPHENE MAY INCREASE PLASMA PHENOBARBITONE LEVELS BY 8-25%. METHADONE PLASMA LEVELS WERE REDUCED. PETHIDINE HAS ADDITIVE SEDATIVE ACTION.
ANTIARRHYTHMICS :- DISOPYRAMIDE , QUINIDINE & LIGNOCAIN CLEARENCE IS INCREASED.
ANTIBACTERIALS :- CHLORAMPHENICOL INCREASE PLASMA CLEARANCE OF PHENOBARBITOBE & VICE VERSA. PHENOBARBITONE INCREASES DOXYCYCLINE METABOLISM.
ANTIDEPRESSANTS :- ANTIDEPRESSANTS MAY ANTAGONISE THE ANTIEPILEPTIC ACTIVITY OF PHENOBARBITONE & OTHER ANTIEPILEPTICS BY LOWERING THE CONVULSIVE THRESHOLD.
HYPERICUM INCREASES PHENOBARBITONE METABOLISM ALSO.
ANTIEPILEPTICS :- SOD VALPROATE INCREASES PHENOBARBITONE PLASMA CONC. BY 17-48%, WHILE PHENOBARBITONE REDUCES VALPROATE SERUM CONC BY INCREASING ITS METABOLISM.
PHENYTOIN IN SOME PTS CAUSE RISE IN PLASMA CONC OF PHENOBARBITONE. PHENOBARBITONE WHILE REDUCES SERUM PHENYTOIN CONC BY INDUCING ITS METABOLISM BUT IN BOTH CASES NO DOSE CHANGE IS REQUIRED.
ANTIFUNGALS :- GRISEOFULVIN ORAL ABSORPTION IS REDUCED WHILE ITRACONAZOLE METABOLISM IS INDUCED CAUSING LOWER PLASMA LEVELS.
ANTIPPROTOZOALS :- METRONIDAZOLE METABOLISM IN ENHANCED.
ANTIPSYCHOTICS :- AS WITH ALL ANTIEPILEPTICS, ANTIPSYCHOTICS MAY ANTAGONISE ANTIEPILEPTIC ACTIVITY OF PHENOBARBITONE BY LOWERING THE CONVULSIVE THRESHOLD.
ANTIVIRALS :- HIV PROTEASE INHIBITORS SERUM LEVEL IS REDUCED DUE TO ENHANCED METABOLISM.
BETABLOCKERS :- PLASMA CONC IS REDUCED DUE TO INCREASED METABOLISM.
CALCIUM CHANNEL BLOCKERS :- BOTH NIFEDIPINE & VERAPAMIL PLASMA LEVELS ARE REDUCED DUE TO INCREASED METABOLISM.
CARDIAC GLYCOSIDES :- DIGOXIN LEVELS ARE GREATLY REDUCED DUE TO INCREASED METABOLISM.
CYCLOSPORIN : PLASMA LEVELS ARE REDUCED
CORTICOSTEROIDS :- PLASMA LEVELS ARE REDUCED DUE TO INCREASED METABOLISM
DIURETICS _ FRUSEMIDE INCREASED SERUM PHENOBARBITONE LEVELS
THYROID HORMONE :- PLASMA CONC IS REDUCED
VACCINE :- INFLUENZA VACCINE CAN CAUSE PROLONGES RISE IN PLASMA LEVELS IN SOME PTS.