Chlormethiazole is a sedative and anticonvulsive drug used in the treatment of alcohol withdrawal. Because it had been reported that chlormethiazole inhibits the alcohol-inducible cytochrome P450 2E1 in rat liver, we investigated the in vivo and in vitro effect of this drug on cytochrome P450 2E1 in human beings. The activity of this cytochrome was assessed using chlorzoxazone as a probe. The 6-hydroxychlorzoxazone- chlorzoxazone blood concentration ratio, reflecting the cytochrome P450 2E1 activity, was determined in 10 controls and in 24 alcoholic patients who had entered a hospital for detoxification. Alcoholic patients were administered either chlormethiazole (1.3-2.3 g/d) or chlorazepate (100-300 mg/d) as a sedative. Cytochrome P450 2E1 activity was significantly increased in alcoholic patients treated with chlorazepate (1.16 ± 0.40 vs. 0.27 ± 0.03, P < .05). In contrast, chlormethiazole treatment inhibited chlorzoxazone hydroxylation almost totally (0.046 ± 0.03, P < .001). After 7-14 days of ethanol withdrawal, alcoholic patients treated with chlorazepate had ratio values similar to those of controls (0.31 ± 0.05), whereas values from alcoholic patients treated with chlormethiazole remained low (0.049 ± 0.01) even though chlormethiazole doses were gradually decreased. Pharmacokinetic studies in controls showed that chlormethiazole-mediated inhibition was present even when chlormethiazole was not detectable in the blood. In addition, the effect of chlormethiazole on cytochrome P450 2E1 was studied in vitro using human liver microsomes. Dixon plot analyses showed a noncompetitive inhibition (Ki = 12 µmol/L). These data clearly show that chlormethiazole is an efficient inhibitor of chlorzoxazone metabolism and thus of cytochrome P450 2E1 activity in human beings. Because cytochrome P450 2E1 induction after chronic ethanol consumption has detrimental effects on the liver through free radical formation, treatment of alcohol detoxification with chlormethiazole may be beneficial.