Isoniazid is an approved antibiotic, which, in combination with other drugs, is the standard treatment for tuberculosis. In the United States, it is also commonly used as tuberculosis prophylaxis, although this is not common in the United Kingdom.

Isoniazid is also effective prophylaxis against tuberculosis in people with HIV, both in preventing infection and in preventing relapse after infection^[1][2]. This may be a useful preventive treatment for HIV-positive patients in the developing world, where antiretroviral therapy can be unavailable, but where tuberculosis prevalence rates are high^[3].

Isoniazid comes as tablets, an elixir to be drunk or a liquid for intravenous injection. Its side-effects can include rash, fever, jaundice, peripheral neuropathy (nerve damage) and hepatitis. The risk of liver problems may be less than commonly believed: a literature search of published data and a survey sent to tuberculosis officers across the United States found that isoniazid was very safe especially when monitoring guidelines were followed, with no increase in the rate of fatal liver toxicity above that seen in the general population^[4].

Stomach upsets from isoniazid are reduced if it is taken with food. Supplements of vitamin B₆ may help to reduce the risk of neuropathy and avoiding alcohol may help to minimise the chances of liver problems. Aluminium-containing antacids or laxatives should be avoided, as they reduce the amount of isoniazid that is absorbed from the gut.

Isoniazid does not interact with any currently available protease inhibitors or non-nucleoside reverse transcriptase inhibitors (NNRTIs).

Isoniazid is available as a generic (non-brand name) product. It is sometimes abbreviated to INH.