Isoniazid (INH), introduced in the 1950s, continues to be one of the most safe and cost-effective agents used to treat tuberculosis (9). Because of the increasing incidence of tuberculosis that has resulted from the coepidemic of human immunodeficiency virus infection (32), the use of INH in the prevention (9, 17) and treatment (9) of tuberculosis is on the rise. INH is often given chronically to critically ill patients who are on multiple medications to treat tuberculosis and AIDS-related mycobacterial disease (18, 45), raising the potential for adverse drug-drug interactions.In humans, INH has been found to decrease the clearance of several drugs, including phenytoin (see, e.g., references 3, 24, and 30), carbamazepine (54, 58, 59), diazepam (36), triazolam (37), vincristine (7), primidone (48), and acetaminophen (10, 35), sufficiently to require a reduction of the dose or discontinuation of INH. Upon consideration of the primary metabolic pathways of the drugs affected and the enzymes catalyzing them, many of these drug-drug interactions appear to be attributable to pharmacokinetic changes that can be understood in terms of alterations of multiple hepatic drug metabolic pathways catalyzed by the cytochrome P450 (CYP450) system (2). In fact, the ability of INH to inhibit the hepatic CYP450 system in vitro in rat liver microsomes has been described (25,34). In animals (6), it has been shown that INH effectively inhibits para-hydroxylation of phenytoin in vivo. Despite the strong link between INH drug interactions and inhibition of the CYP450 system and despite its widespread use for more than 4 decades, our knowledge is incomplete with respect to which specific CYP450 isoforms are inhibited, making prediction of drug interactions with INH difficult. The only isoform that has been studied in detail in human and animal models is CYP2E1, for which INH has been reported to have a biphasic effect (induction and inhibition) (8). This property of INH may explain the increased risk of hepatotoxicity of coadministered compounds (e.g., ethanol and acetaminophen) (10,33,35), but it is unlikely to explain the clinically documented interactions with other drugs, as most of them appear to be catalyzed by CYP isoforms other than CYP2E1 (e.g., phenytoin by CYP2C9 and CYP2C19 and carbamazepine by CYP3A and CYP2C8 [2,28]).In order to enable physicians to improve predictions about which drugs might interact with INH, the present study was undertaken to evaluate the inhibitory potency of INH on six common human drug-metabolizing CYP450 isoforms in vitro.