SETTING: In 10 healthy subjects, the influence of acute alcohol intake on the pharmacokinetics of isoniazid (INH) was studied. OBJECTIVE: To test the hypothesis that alcohol increases the conversion of INH by acetylation into its metabolite acetylisoniazid. DESIGN: In a crossover design, an oral dose of 300 mg INH was administered on 2 separate days, 14 days apart, with or without alcohol to a serum alcohol of about 21 mmol/l (1 g/l) maintained for 12 h. RESULTS: Neither the metabolism of INH nor that of acetylisoniazid was changed by acute alcohol intake. CONCLUSION: Acute alcohol intake has no impact on the conversion of INH to its metabolite acetylisoniazid, which is catalysed by the enzyme N-acetyltranferase. Accordingly, a metabolic effect of acute alcohol intake on INH metabolism probably contributes little to the therapeutic failure of anti-tuberculosis treatment among alcoholics.