Isoniazid is a bactericidal agent active against organisms of the genus Mycobacterium, specifically M. tuberculosis, M. bovis and M. kansasii. Isoniazid is recommended for all forms of tuberculosis in which organisms are susceptible. Isoniazid is a prodrug and must be activated by bacterial catalase. Isoniazid inhibits InhA, the enoyl reductase from Mycobacterium tuberculosis, by forming a covalent adduct with the NAD cofactor. The most frequent adverse reactions to isoniazid are those affecting the nervous system and the liver.

Isobutol (trade name ISOETAM) is a salt of ethambutol and isoniazid methanesulfonate with tuberculostatic and bacteriostatic action. The purpose of this combination is to keep the strong anti-microbic activity of isoniazid in the manner where ethambutol would delay the appearance of resistance.