A study of the effects of rifampicin on isoniazid metabolism in human volunteer subjects.

1 The effect of rifampicin on the metabolism of isoniazid in human volunteer subjects has been investigated.2 The urinary metabolites of isoniazid after a single dose and after six daily doses of isoniazid plus rifampicin were examined.3 The isoniazid-rifampicin combination clearly increased the ratio of urinary 6-β-hydroxycortisol to 17-hydroxycorticosteroids, indicating that induction of the microsomal enzymes had occurred.4 However, no significant changes in the urinary metabolites of isoniazid were detected, and therefore it is not possible to predict the effect of rifampicin on isoniazid hepatotoxicity.