INCREASES EFFECT OF PHENYTOIN BECAUSE OF DECREASED EXCRETION. CAUSE HYPERPYREXIA, TREMOR AND DEATH WITH ALCOHOL. PREDNISOLONE DECREASES INH LEVEL. ANTAGONISES HYPOGLYCAEMIC ACTION OF INSULIN. IT INCREASES VIT B6 REQUIREMENT. REDUCES ALPRAZOLAM & OTHER BENZODIAZEPINES CLEARENCE SO REDUCE ALPRAZOLAM DOSE BY 1/3. BECAUSE ISONIAZID HAS SOME MONOAMINE OXIDASE INHIBITING ACTIVITY, AN INTERACTION WITH TYRAMINE-CONTAINING FOODS (CHEESE, RED WINE, BROAD BEANS, YEAST EXTRACTS, BEAR, PICKLED HERRING, CHICKEN LIVER & FERMENTED PRODUCTS) MAY OCCUR. DIAMINE OXIDASE MAY ALSO BE INHIBITED, CAUSING EXAGGERATED RESPONSE (EG, HEADACHE, SWEATING, PALPITATIONS, FLUSHING, HYPOTENSION) TO FOODS CONTAINING HISTAMINE (EG, SKIPJACK, TUNA, OTHER TROPICAL FISH). ISONIAZID HAS BEEN REPORTED TO INHIBIT THE METABOLISM OF THE FOLLOWING DRUGS: ANTICONVULSANTS (EG, CARBAMAZEPINE, PHENYTOIN, PRIMIDONE, VALPROIC ACID), BENZODIAZEPINES (EG, DIAZEPAM), HALOPERIDOL, KETOCONAZOLE, THEOPHYLLINE, AND WARFARIN. CONCOMITANT ANTACID ADMINISTRATION MAY REDUCE THE ABSORPTION OF ISONIAZID. INGESTION WITH FOOD MAY ALSO REDUCE THE ABSORPTION OF ISONIAZID. EFFICACY INCREASED BY PAS. PARA-AMINOSALICYLIC ACID MAY INCREASE THE PLASMA CONCENTRATION AND ELIMINATION HALF-LIFE OF ISONIAZID BY COMPETITION OF ACETYLATING ENZYMES. FAST ACETYLATION OF ISONIAZID MAY PRODUCE HIGH CONCENTRATIONS OF HYDRAZINE WHICH FACILITATE DEFLORINATION OF ENFLURANE. RENAL FUNCTION SHOULD BE MONITORED IN PATIENTS RECEIVING BOTH ISONIAZID AND ENFLURANE. THE CNS EFFECTS OF MEPERIDINE (DROWSINESS), CYCLOSERINE (DIZZINESS, DROWSINESS), AND DISULFIRAM (ACUTE BEHAVIORAL AND COORDINATION CHANGES) MAY BE EXAGGERATED WHEN CONCOMITANT ISONIAZID IS GIVEN. LEVODOPA ADMINISTRATION MAY PRODUCE SYMPTOMS OF EXCESS CATECHOLAMINE STIMULATION (AGITATION, FLUSHING, PALPITATIONS) OR LACK OF LEVODOPA EFFECT.