Isoniazid is bactericidal to rapidly dividing mycobacteria, but is bacteriostatic if the mycobacteria are slow-growing.[40] It inhibits the cytochrome P450 system and hence acts as a source of free radicals.
Isoniazid is often used to treat latent and active tuberculosis infections. In persons with isoniazid-sensitive Mycobacterium tuberculosis infection, drug regimens based on isoniazid are usually effective when persons adhere to the prescribed treatment.
Isoniazid reaches therapeutic concentrations in serum, cerebrospinal fluid, and within caseous granulomas. It is metabolized in the liver via acetylation into acetylhydrazine.