Isoniazid (INH) is a first-line, bactericidal antitubercular agent, considered a cornerstone of tuberculosis (TB) treatment and prevention 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 effective against actively dividing Mycobacterium tuberculosis organisms and is a key component of the Directly Observed Treatment, Short-course (DOTS) strategy.
Adult: Active TB: 5 mg/kg (usually 300 mg) once daily or 15 mg/kg (max 900 mg) 2-3 times per week under DOTS. LTBI: 5 mg/kg (max 300 mg) once daily for 6-9 months, or 900 mg twice weekly for 3 months.
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, may be taken with food, acknowledging reduced absorption. Pyridoxine (10-25 mg daily) should be co-administered. For LTBI, adherence is critical; use pill boxes or reminders.
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) enzyme. This inhibition disrupts the synthesis of mycolic acids, which are long-chain fatty acids critical for the formation and integrity of the mycobacterial cell wall.
Pregnancy: Pregnancy Category C (US FDA). Considered safe and essential for treatment of active TB in pregnancy. The benefit outweighs risk. Pyridoxine supplementation (25 mg/day) is mandatory. Monitor LFTs.
Driving: May cause dizziness or vertigo. Caution patients about operating machinery or driving until they know how the drug affects them.
| Rifampicin | Increases risk of hepatotoxicity. Also induces INH metabolism, potentially lowering levels. | Major |
| Pyrazinamide | Synergistic hepatotoxicity risk. | Major |
| Phenytoin / Carbamazepine | INH inhibits their metabolism, leading to increased levels and toxicity (ataxia, nystagmus). | Major |
| Warfarin | INH may potentiate anticoagulant effect; monitor INR closely. | Moderate |
| Ketoconazole / Itraconazole | INH may decrease their plasma levels via CYP induction. | Moderate |
| Antacids (Aluminum-containing) | May decrease INH absorption. Separate administration by at least 2 hours. | Moderate |
| Acetaminophen (Paracetamol) | Increased risk of hepatotoxicity, especially in overdose settings. | Moderate |
| Disulfiram | Increased risk of neuropsychiatric reactions. | Moderate |
| Diazepam, Triazolam | INH may inhibit their metabolism, increasing sedation. | Moderate |
Same composition (Isoniazid (300mg)), different brands: