Prescribing Guides
Isoniazid Prescribing Guide for Indian Clinical Practice
Isoniazid prescribing guide for Indian clinicians covering INH pharmacology, acetylator status, peripheral neuropathy prevention and NEET PG high-yield points.
MedNext Editorial Team | 2026-08-01 | 8 min read
Isoniazid Prescribing Guide for Indian Clinical Practice
Isoniazid prescribing guide for Indian clinicians covering INH pharmacology, acetylator status, peripheral neuropathy prevention and NEET PG high-yield points.
Isoniazid (INH) is the most potent bactericidal drug against actively growing M. tuberculosis and is used in every first-line TB regimen. India's high TB burden makes INH pharmacology one of the most tested topics in NEET PG.
Clinical Pharmacology Overview
Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase enzyme KatG. The activated form inhibits InhA, an enoyl-ACP reductase involved in mycolic acid synthesis, which is essential for the mycobacterial cell wall. INH is bactericidal against actively multiplying bacilli but only bacteriostatic against semi-dormant organisms.
Pharmacokinetics: Rapidly and completely absorbed orally (bioavailability nearly 100% on empty stomach, reduced by food). Low protein binding (10%). Distributed to all body fluids including CSF. Metabolised by N-acetyltransferase 2 (NAT2) -- genetically polymorphic (slow and fast acetylators). Half-life is 1-3 hours (fast acetylators) or 3-5 hours (slow acetylators). Excreted renally.
Indian Brand Names
- INH (generic, government supply)
- Isokin (Lupin)
- R-Cinex (Lupin) -- rifampicin + isoniazid FDC
- Various FDC formulations under NTEP supply
- Isoniazid IP (multiple manufacturers)
Approved Indications in Indian Clinical Practice
- Drug-sensitive pulmonary and extrapulmonary tuberculosis (as part of multidrug regimen)
- Latent TB infection prophylaxis (isoniazid preventive therapy / IPT)
- TB-HIV co-infection prophylaxis
- Contact prophylaxis for household contacts of active TB cases
Dosing in Adults
Treatment of active TB (NTEP): 5 mg/kg/day (max 300 mg) as part of daily FDC regimen.
Intensive phase: isoniazid + rifampicin + pyrazinamide + ethambutol for 2 months.
Continuation phase: isoniazid + rifampicin for 4 months.
LTBI prophylaxis: 5 mg/kg/day (max 300 mg) for 6-9 months.
Always co-prescribe pyridoxine (vitamin B6) 10 mg/day to prevent peripheral neuropathy.
Administer on an empty stomach for optimal absorption (1 hour before or 2 hours after meals).
Important Drug Interactions
- Rifampicin: additive hepatotoxicity (acetylhydrazine accumulation). Monitor LFTs closely.
- Phenytoin: INH inhibits CYP2C19, raising phenytoin levels -- toxicity risk, especially in slow acetylators
- Carbamazepine: INH increases carbamazepine levels
- Paracetamol: INH induces CYP2E1, increasing formation of the hepatotoxic NAPQI metabolite of paracetamol
- Aluminium-containing antacids: reduce INH absorption
- Tyramine-containing foods (cheese, wine): INH has weak MAO inhibitor activity -- rare hypertensive reactions
- Disulfiram: additive neurotoxicity
- Ketoconazole: INH may reduce ketoconazole levels
Side Effects and Monitoring
Common: peripheral neuropathy (prevented by pyridoxine), hepatitis, GI upset.
Serious: hepatotoxicity (INH hepatitis -- can be fatal; risk increases with age, alcohol, and concurrent rifampicin/pyrazinamide), peripheral neuropathy, optic neuritis, seizures (in overdose), drug-induced lupus, sideroblastic anaemia (pyridoxine-responsive), pellagra (niacin deficiency).
Hepatotoxicity risk: less than 1% under age 20, approximately 2% age 35-49, more than 3% above age 50.
Monitoring: baseline LFTs. Monthly LFTs for the first 3 months (or sooner if symptomatic). Stop INH if ALT exceeds 5x ULN (asymptomatic) or 3x ULN with symptoms. Educate patient about symptoms of hepatitis (nausea, anorexia, dark urine, jaundice).
Special Populations
Pregnancy: Category C. Considered safe and essential for TB treatment in pregnancy. The risk of untreated TB far exceeds drug risk. Always supplement with pyridoxine 25-50 mg/day in pregnancy.
Renal impairment: Dose reduction generally not needed for standard regimen in mild-moderate CKD. In severe renal impairment (GFR < 30), some guidelines suggest reducing to 200 mg/day and monitoring levels.
Hepatic impairment: Use with caution. Increased hepatotoxicity risk. Modified regimens may be needed in severe liver disease.
Elderly: Higher hepatotoxicity risk. Monitor LFTs more frequently.
HIV co-infection: Standard INH dosing. IPT for 6-9 months recommended for all PLHIV without active TB.
NEET PG High-Yield Points
- Prodrug activated by KatG (catalase-peroxidase)
- Inhibits InhA (enoyl-ACP reductase) -- blocks mycolic acid synthesis
- Peripheral neuropathy: pyridoxine (B6) deficiency -- prevented by supplementation
- NAT2 polymorphism: slow vs fast acetylators -- affects toxicity profile
- Hepatotoxicity: via acetylhydrazine metabolite, risk increases with age and alcohol
- Inhibits CYP2C19 (phenytoin interaction) and induces CYP2E1 (paracetamol interaction)
- Drug-induced SLE with anti-histone antibodies -- more common in slow acetylators
- MDR-TB = resistance to isoniazid + rifampicin
- KatG mutation = most common mechanism of INH resistance
- Sideroblastic anaemia: pyridoxine-responsive
Indian Regulatory Status
Isoniazid is a Schedule H drug under the Drugs and Cosmetics Act. It is included in the NLEM of India. Under the NTEP, isoniazid is available only as part of fixed-dose combinations (with rifampicin, pyrazinamide, ethambutol) to prevent monotherapy and resistance development. Single-drug isoniazid is available for LTBI prophylaxis. Pyridoxine co-prescription is recommended in all NTEP treatment regimens.
Frequently Asked Questions
Why must pyridoxine be given with isoniazid?
Isoniazid competes with pyridoxal phosphate (active form of vitamin B6) and increases its renal excretion, causing pyridoxine deficiency. This manifests as peripheral neuropathy (symmetric, sensory, stocking-glove distribution). Pyridoxine 10 mg/day is routinely co-prescribed with INH in India to prevent this.
What is the significance of acetylator status?
Isoniazid is metabolised by N-acetyltransferase 2 (NAT2). Slow acetylators (approximately 40-60% of Indian population) have higher drug levels and greater risk of peripheral neuropathy and hepatotoxicity. Fast acetylators may have subtherapeutic levels but are more likely to produce hepatotoxic metabolites (acetylhydrazine).
What is the dose of INH for latent TB treatment?
For LTBI prophylaxis: INH 5 mg/kg/day (max 300 mg) for 6-9 months, with pyridoxine. Alternative: INH + rifapentine (3HP regimen) weekly for 12 weeks. NTEP provides IPT (isoniazid preventive therapy) for eligible contacts and HIV-positive individuals.
Can isoniazid cause lupus-like syndrome?
Yes. INH can cause drug-induced SLE, especially in slow acetylators. It presents with arthralgia, fever, rash, and positive anti-histone antibodies (anti-dsDNA is typically negative, distinguishing it from idiopathic SLE). The condition resolves on drug withdrawal.
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