Section 15 · Monograph
Isoniazid
Reviewed Nov 2026FDA label
INHAntimycobacterial
- Forms
- Tab: 100, 300mg | IV: 100mg/mL | Syrup: 50mg/5mL
- Dose
- TB meningitis: 5mg/kg/day (max 300mg) with pyridoxine; part of a 4-drug regimen
- MOA
- Inhibits mycolic acid synthesis (InhA); good CSF penetration
Black Box Warning
- Severe and sometimes fatal hepatitis may occur at any time during treatment, with risk increasing with age and alcohol use; monitor liver function monthly and educate patients on symptoms; discontinue if clinical signs of hepatitis develop
Indications
- Treatment of all forms of tuberculosis including TB meningitis; prevention of tuberculosisFDA-labeled
Mechanism of Action
- Prodrug activated by KatG that inhibits mycolic acid synthesis (InhA); bactericidal against replicating mycobacteria with excellent CSF penetration
Full Dosing
Adult / usual
- Give pyridoxine 25-50mg/day to prevent peripheral neuropathy and neurotoxicity, especially with diabetes, alcohol use, malnutrition, HIV or pregnancy
- TB meningitis is typically treated with four drugs initially, then isoniazid plus rifampin for a total of 9-12 months with adjunctive dexamethasone; follow current ATS/CDC/IDSA or WHO guidance
Pediatric
- Adults 5mg/kg/day (max 300mg) PO or IV; children 10-15mg/kg/day (max 300mg)
Contraindications
- History of isoniazid-associated hepatic injury or severe adverse reactions (drug fever, chills, arthritis)
- Acute liver disease of any etiology
Drug Interactions
- Inhibits CYP enzymes: raises phenytoin, carbamazepine, valproate and diazepam levels — monitor levels and toxicity closely
- Acetaminophen, alcohol and rifampin increase hepatotoxicity
- Disulfiram: psychosis reports; antacids reduce absorption
Adverse Effects
- Hepatitis
- Peripheral neuropathy and optic neuritis
- Seizures and psychosis (especially overdose)
- Drug-induced lupus, rash
Monitoring Parameters
- Baseline and monthly liver enzymes (more often with risk factors)
- Neurologic and visual symptoms
- Antiepileptic drug levels when co-prescribed
Pregnancy & Lactation
- Considered safe with pyridoxine supplementation
- Compatible with breastfeeding; supplement pyridoxine for infant and mother