Isoniazid (Isoniazid)
Also sold as: INH, Isokin, Pulmoxyl, Solonex
Pregnancy
Cat C
Lactation
Caution
Schedule
H1
Forms
Tablet 100mg +3
Indications
- Active tuberculosis (first-line DOTS regimen)
- Latent tuberculosis infection (LTBI) prophylaxis
- TB chemoprophylaxis in HIV-positive individuals
- TB prophylaxis in close contacts of active TB cases
Adult Dosing
Active tuberculosis (DOTS intensive phase)
5 mg/kg OD (max 300 mg)
Daily for 2 months (intensive phase) then 4 months (continuation)
Always give with pyridoxine 25–50 mg daily to prevent peripheral neuropathy; take on empty stomach
Latent TB prophylaxis (LTBI)
300 mg OD
6 months (HIV-negative) or 9 months (HIV-positive)
Monthly LFT monitoring; give with pyridoxine
Pediatric Dosing
Pyridoxine supplementation recommended in malnourished children, breastfed infants, or those with HIV; syrup available for young children
Renal Dose Adjustment
| CrCl / eGFR | Dose Adjustment |
|---|---|
| CrCl ≥30 mL/min | Standard dose |
| CrCl <30 mL/min or haemodialysis | Standard dose; administer after dialysis on dialysis days; increase pyridoxine to 50 mg/day |
Hepatic Adjustment
Contraindicated in acute hepatic failure. Use with caution in pre-existing liver disease; slow acetylators and those with hepatic disease at higher risk. Monitor LFTs monthly. Stop if ALT >3× ULN symptomatic or >5× ULN asymptomatic.
Pregnancy & Lactation
Pregnancy: Category C
Generally safe during pregnancy — WHO recommends standard TB treatment including isoniazid throughout pregnancy. Give pyridoxine 25 mg/day to all pregnant women on isoniazid. Risk of untreated TB far exceeds drug risk.
Lactation: Caution
Excreted in breast milk in small concentrations. WHO supports continued breastfeeding during TB treatment. Give pyridoxine to the mother; monitor infant for peripheral neuropathy signs.
Top Drug Interactions
| Interacting Drug | Effect | Severity |
|---|---|---|
| Phenytoin | Isoniazid inhibits CYP2C9 — significantly increases phenytoin levels; may cause phenytoin toxicity (nystagmus, ataxia, confusion); monitor phenytoin levels closely | Major |
| Carbamazepine | Isoniazid inhibits CYP3A4 — increases carbamazepine levels; risk of carbamazepine toxicity; monitor levels and reduce dose if needed | Major |
| Alcohol | Increases risk of isoniazid hepatotoxicity significantly; also increases risk of isoniazid-induced peripheral neuropathy; advise complete abstinence | Major |
| Rifampicin | Additive hepatotoxicity — used together in standard TB regimen; monitor LFTs monthly; risk higher with slow acetylators | Moderate |
| Antacids (aluminium-containing) | Reduces isoniazid absorption; take isoniazid at least 1 hour before antacids | Minor |
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Side Effects
Common
- Peripheral neuropathy (tingling, numbness, burning in hands/feet) — prevented by pyridoxine co-administration
- Nausea, vomiting, epigastric discomfort
- Mild transaminase elevation (often asymptomatic — monitor)
- Headache, dizziness
- Rash (mild)
Serious / Discontinue If
- Hepatotoxicity — ranges from asymptomatic transaminase elevation to fulminant hepatic failure; risk increases with age, alcohol use, slow acetylator status, pre-existing liver disease
- Severe peripheral neuropathy (if pyridoxine not given, especially in high-risk patients)
- Drug-induced lupus erythematosus (anti-histone antibodies)
- Agranulocytosis, aplastic anaemia (rare)
- Psychosis, encephalopathy (rare — more likely with pyridoxine deficiency)
- Optic neuritis (rare)
Contraindications
- Acute liver failure or severe hepatic impairment
- Previous isoniazid-induced hepatitis
- Hypersensitivity to isoniazid
- Active hepatic disease with ALT >3× ULN
Available Indian Brands
| Brand | Manufacturer | Price (approx) |
|---|---|---|
| Solonex 300mg | Lupin | ₹18/10 tab |
| Isokin 300mg | IPCA | ₹15/10 tab |
Monitoring Required
- Baseline LFTs before starting (ALT, AST, bilirubin)
- Monthly LFTs during treatment — especially in patients >35 years, with liver disease, alcohol use, or on rifampicin
- Assess for peripheral neuropathy symptoms at each visit
- Acetylator status consideration (slow acetylators at higher hepatotoxicity risk) — not routinely tested but important to know
- FBC if prolonged treatment or immunocompromised
Patient Counseling Points
- Take pyridoxine (vitamin B6) every day with isoniazid — this prevents nerve damage in hands and feet
- Take on an empty stomach for best absorption; if stomach upset is severe, can take with a small amount of food
- Avoid all alcohol during TB treatment — it significantly increases the risk of liver damage
- Report immediately: yellowing of eyes or skin, dark urine, loss of appetite, nausea/vomiting, stomach pain — these may be signs of liver damage
- Report tingling, numbness, or burning in hands or feet — tell your doctor promptly
- Never stop this medicine without telling your doctor — incomplete treatment leads to drug-resistant TB
- If you are an epileptic patient on phenytoin or carbamazepine, your doctor needs to monitor those drug levels closely
- Complete the full course — 6 months minimum for active TB
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Clinically reviewed by: Dr. Anand Kumar, MD (Pulmonology), Chest Physician, TB & Chest Hospital, New Delhi
Last reviewed: 2026-04-30
References
- WHO Treatment of Drug-Susceptible Tuberculosis Guidelines, 2022
- RNTCP DOTS Guidelines, Central TB Division, MoHFW India
- ATS/CDC/IDSA Statement: Treatment of Tuberculosis, 2003 (updated 2016)
- Micromedex Drug Interactions — Isoniazid, 2024
- BNF (British National Formulary) — Isoniazid monograph, 2024