Isoniazid, also known as isonicotinic acid hydrazide (INH), is an antibiotic used for the treatment of tuberculosis. For active tuberculosis it is often used together with rifampicin, pyrazinamide, and either streptomycin or ethambutol. For latent tuberculosis it is often used by itself. It may also be used for atypical types of mycobacteria, such as M. avium, M. kansasii, and M. xenopi. It is usually taken by mouth but may be used by injection into muscle.

Common side effects include increased blood levels of liver enzymes and numbness in the hands and feet. Serious side effects may include liver inflammation and acute liver failure. It is unclear if use during pregnancy is safe for the baby. Use during breastfeeding is likely safe. Pyridoxine may be given to reduce the risk of side effects. Isoniazid works in part by disrupting the formation of the bacteria's cell wall which results in cell death.

Isoniazid was first made in 1952. It is on the World Health Organization's List of Essential Medicines. The World Health Organization classifies isoniazid as critically important for human medicine. Isoniazid is available as a generic medication.