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Anti-Tuberculosis Antibiotic Pregnancy: Compatible — standard component of TB treatment in pregnancy; pyridoxine essential

Isoniazid

Brand names: Rimifon

Adult dose

Dose: TB treatment: 300 mg OD. Latent TB (LTBI): 300 mg OD × 6 months (+ pyridoxine 10 mg OD)
Route: Oral
Frequency: Once daily
Max: 300 mg/day
Component of standard RHZE TB regimen. Always give pyridoxine 10 mg OD with isoniazid to prevent peripheral neuropathy. Metaboliser status (fast vs slow acetylators) affects levels.

Paediatric dose

Dose: 10 mg/kg
Route: Oral
Frequency: Once daily
Max: 300 mg/day
Concentration: 10 mg/ml
BNF for Children: 10 mg/kg OD (max 300 mg). Always co-prescribe pyridoxine 5–10 mg OD in children (peripheral neuropathy prevention). Neonates: 5–10 mg/kg OD. Source: BNF for Children 2024; NICE NG33 TB

Dose adjustments

Renal

No dose adjustment usually required; slow acetylators may accumulate

Hepatic

Significant risk of hepatotoxicity — avoid in acute hepatic disease; monitor LFTs closely

Paediatric weight-based calculator

BNF for Children: 10 mg/kg OD (max 300 mg). Always co-prescribe pyridoxine 5–10 mg OD in children (peripheral neuropathy prevention). Neonates: 5–10 mg/kg OD. Source: BNF for Children 2024; NICE NG33 TB

Clinical pearls

  • Pyridoxine 10 mg OD with every isoniazid prescription — peripheral neuropathy prevention
  • LFTs at baseline; repeat if symptomatic (nausea, jaundice, vomiting) — drug-induced hepatitis is reversible if caught early
  • Fast vs slow acetylator phenotype: fast acetylators have lower plasma levels (less effective) and paradoxically less peripheral neuropathy; slow acetylators: higher levels, more neuropathy
  • LTBI: 6 months isoniazid or 3 months isoniazid + rifampicin or 3 months rifampicin alone (NICE)

Contraindications

  • Drug-induced liver disease
  • Acute hepatitis

Side effects

  • Peripheral neuropathy (pyridoxine-deficiency mediated — give pyridoxine prophylactically)
  • Hepatotoxicity (dose-related; more common in elderly, alcohol users, fast acetylators)
  • Lupus-like syndrome
  • Seizures (rare — depletes pyridoxine)
  • Drug-induced liver injury (dose-related)

Interactions

  • Phenytoin — isoniazid significantly increases phenytoin levels (monitor levels, reduce phenytoin dose)
  • Carbamazepine — increased carbamazepine toxicity
  • Rifampicin — combined hepatotoxicity risk; enzyme interaction complex
  • Alcohol — increased hepatotoxicity

Monitoring

  • LFTs (baseline, monthly)
  • Peripheral neuropathy symptoms
  • Pyridoxine supplementation confirmed
  • Seizure risk assessment

Reference: BNFc; BNF; NICE NG33 TB; British Thoracic Society TB Guidelines. Verify against your local formulary and the latest BNF before prescribing.

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