Isoniazid (INH) is a first-line, bactericidal antitubercular agent, considered a cornerstone of tuberculosis (TB) treatment and prophylaxis in India. It is a prodrug activated by the bacterial catalase-peroxidase enzyme (KatG), leading to inhibition of mycolic acid synthesis, essential for the mycobacterial cell wall. It is highly specific for Mycobacterium tuberculosis, with some activity against M. kansasii. In the Indian context, it is a Schedule H1 drug, critical for both the National Tuberculosis Elimination Programme (NTEP) and private sector management of TB.
Adult: Active TB: 5 mg/kg (usually 300 mg) once daily. LTBI Prophylaxis: 5 mg/kg (max 300 mg) once daily for 6-9 months. Under NTEP, weight-band based Fixed Dose Combinations (FDCs) are used.
Note: Take on an empty stomach, at least 1 hour before or 2 hours after meals, with a full glass of water to maximize absorption. If GI upset occurs, can be taken with a light, low-fat meal. MUST be co-administered with Pyridoxine (Vitamin B6) 10-25 mg daily. Adherence to the full course is critical.
Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase enzyme (KatG). The activated form forms a covalent adduct with NAD+ and NADP+, which then potently inhibits the enoyl-acyl carrier protein reductase (InhA) in the FAS-II system. This blockade disrupts the synthesis of mycolic acids, which are long-chain fatty acids critical for the integrity and impermeability of the mycobacterial cell wall.
Pregnancy: US FDA Category C. Considered generally safe and recommended for treatment of active TB and LTBI in pregnancy. The benefit of treating TB far outweighs the risk. Pyridoxine supplementation (25 mg/day) is crucial. Part of standard NTEP regimens for pregnant women.
Driving: Usually no effect. However, if symptoms of CNS toxicity like dizziness, drowsiness, or optic neuritis occur, patients should avoid driving or operating machinery.
| Rifampicin | Induces INH metabolism, may slightly decrease INH levels but increases hepatotoxicity risk synergistically. | Major |
| Phenytoin / Carbamazepine | INH inhibits their metabolism, increasing serum levels and risk of toxicity (ataxia, nystagmus, drowsiness). | Major |
| Warfarin | INH may potentiate anticoagulant effect by inhibiting warfarin metabolism. Requires frequent INR monitoring. | Major |
| Antacids (Aluminium containing) | May decrease absorption of INH. Administer at least 2 hours apart. | Moderate |
| Ketoconazole / Itraconazole | INH may reduce azole levels. Monitor for antifungal efficacy. | Moderate |
| Acetaminophen (Paracetamol) | Increased risk of hepatotoxicity, especially in overdose. Use with caution at therapeutic doses. | Moderate |
| Disulfiram | Concurrent use may cause coordination difficulties and psychotic reactions. | Moderate |
| Benzodiazepines (e.g., Diazepam) | INH may inhibit metabolism, prolonging sedative effect. | Moderate |