Isoniazid is one of the drugs used in standard, first-line TB therapy for drug-sensitive patients. It was discovered in 1952. It has high early bactericidal activity that kills actively growing bacteria and causes a rapid decrease in sputum bacilli for the first 2 weeks of treatment, then slows down for non-growing bacterial populations.

Isoniazid is a prodrug activated by catalase-peroxidase hemoprotein, KatG. Isoniazid inhibits InhA, a nicotinamide adenine dinucleotide (NADH)-specific enoyl-acyl carrier protein (ACP) reductase involved in fatty acid synthesis. Isoniazid interacts with the cytochrome P450 system, especially CYP2E1, where it shows a biphasic inhibition induction; it causes increases in serum concentrations of various drugs.

Side effects of isoniazid use include hepatitis, the risks of which increase with age and alcohol consumption. Peripheral neuropathy is the most common central nervous system-related toxic effect. It is dose-related, and occurs most often in the malnourished and in those predisposed to neuritis (e.g., alcoholics and diabetics).