Potentiation of acetaminophen hepatotoxicity has previously been associated with a history of alcohol abuse. Presented here is the case of a 21-yr-old Philippino female with rapidly deteriorating hepatic functions. She had been on isoniazid, 300 mg daily, as prophylaxis against tuberculosis due to a positive tuberculin skin test. She took 3.25 g of acetaminophen for abdominal cramping and subsequently had rapid deterioration of liver function manifested by prolongation of the prothrombin time, elevated ammonia, marked elevation of transaminases, and hyperbilirubinemia. Over the course of 1 wk, these values essentially normalized and she was discharged. Isoniazid induces the cytochrome P-450 system, resulting in increased metabolism of acetaminophen, formation of toxic metabolites, depletion of glutathione stores, and subsequent hepatocellular injury. Patients on isoniazid should use caution when taking acetaminophen since the potentially hepatotoxic effects may be amplified due to induction of the cytochrome P-450 system

Standard short-course regimens for tuberculosis are used worldwide with very few problems. Unfortunately, the emergence of multiple drug- resistant tuberculosis in many parts of the world is leading to a diversification of drug regimens and to the use of drugs that are more toxic per se and more likely to interact with others. In addition, the treatment of HIV/AIDS patients with tuberculosis or disease due to Mycobacterium avium-intracellulare complex (MAC) infection with new drugs and multidrug regimens has added to the problem of drug interactions, especially as such patients may often be receiving concomitant treatment for a range of bacterial, fungal and viral infections. In general, there are very few clinically significant interactions between the first-line antituberculosis drugs themselves, although problems of bioavailability, notably of rifampicin (rifampin), have been encountered in the manufacture of combination tablets. Of the first-line drugs used to treat tuberculosis, i.e. rifampicin, isoniazid and pyrazinamide, rifampicin is particularly likely to cause clinically significant drug interactions as it is a potent inducer of the cytochrome P450 enzyme group, which is involved in the metabolism of many drugs, in particular oral contraceptives, corticosteroids, oral anticoagulants and cyclosproin. The use of quinolones to treat multiple drug-resistant tuberculosis and AIDS-related MAC disease raises further problems of drug interactions as, in contrast to rifampicin, these drugs inhibit some cytochrome isoenzymes, leading to reduced metabolism of certain drugs

We report three patients who experienced hepatotoxic reactions in association with acetaminophen ingestion while undergoing treatment for active tuberculosis with isoniazid, rifampin, and other agents. All were young adult women. One patient intentionally took a large amount of acetaminophen and had typical signs and symptoms of acetaminophen overdosage; another took acetaminophen in combination form for a minor upper respiratory illness. She experienced no symptoms. The remaining patient took acetaminophen to ameliorate the symptoms of fever and malaise that were subsequently attributed to tuberculosis. She had the rapid onset of signs and symptoms of isoniazid hepatotoxicity. The patterns of liver function abnormalities were similar: each patient experienced pronounced serum elevations of hepatocellular enzymes with at most only modest rises in those of bilirubin. All antituberculous drugs were withheld until symptoms resolved and laboratory values became normal; then treatment for tuberculosis was resumed without isoniazid and was successfully completed in all three patients. These cases plus similar reports in the literature suggest that isoniazid or rifampin, or both, may potentiate the hepatotoxicity of acetaminophen, perhaps by induction of cytochrome P450 isozymes that oxidize acetaminophen to its toxic metabolites

1. The effect of isoniazid given daily for 7 days on paracetamol (acetaminophen) kinetics and metabolism was studied in 10 healthy volunteers. Paracetamol, 500 mg, was given before isoniazid, on day 7 of isoniazid administration, and 2 days after the last dose of isoniazid. 2. On day 7, isoniazid markedly inhibited the formation clearance of the glutathione and catechol metabolites by 69.7% and 62.2%, respectively. Total paracetamol clearance was lowered by 15.2%. There was no effect of isoniazid on the non-oxidative pathways of paracetamol elimination. 3. Two days after isoniazid was discontinued, paracetamol metabolism had returned to pre-isoniazid values

Isoniazid inhibits the metabolism of several drugs, resulting in clinically significant interactions in some patients. Clinical trials and case reports have documented that isoniazid can cause increased phenytoin and carbamazepine serum concentrations and toxicity. In relatively high doses, isoniazid can also cause increased effect of theophylline and warfarin. Isoniazid inhibits metabolism of selected benzodiazepines and vitamin D. Inhibition of monoamine oxidase and histaminase by isoniazid can cause significant drug-food interactions. Food greatly decreases isoniazid bioavailability. Although probably best recognized as an inhibitor of drug metabolism, isoniazid has a biphasic effect of inhibition-induction on one cytochrome P450 isozyme, CYP2E1, which partially explains the interaction with acetaminophen and increased risk of hepatotoxicity. Continued investigations will likely result in discovery of new isoniazid interactions