Isoniazid is active against susceptible bacteria only when they are undergoing cell division. Susceptible bacteria may undergo 1 or 2 divisions before multiplication is arressted. INH is completely and rapidly absorbed orally and parenterally and peaks in blood levels within 1 to 2 hours. Absorption rate and extent are decreased by food. It passes through the placental barrier and into breast milk in concentrations comparable to those in plasma.
Liver disease can prolong the clearance of isoniazid.
Acetylation rate does not alter the effectiveness of the drug. Slow acetylation may lead to increased blood levels and thus more toxic reactions. Fast acetylators may be more likely to develop hepatitis since hepatotoxicity is caused by the acetylated metabolite of INH, but this remains controversial. However, slow inactivators may develop toxic levels in the presence of impaired renal function. Other Major Uses Isoniazid is used in conjunction with other antituberculosis agents in the treatment of clinical tuberculosis.