Isoniazid is the critical drug in the treatment of tuberculosis. Mycobacteria (including M. tuberculosis) differ from other bacteria in having a cell wall with mycolic acid lipids. Isoniazid inhibits the synthesis of mycolic acid and thus disturbs the cell wall.
The drug is well absorbed orally and is metabolised in the liver by acetylation. Isoniazid can cause fever, hepatitis and other liver diseases and peripheral neuropathy.
Isoniazid is usually given in combination with rifampicin to treat tuberculosis.