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Anti-Tuberculosis Antibiotic Pregnancy: Isoniazid crosses the placenta, so it should only be used in pregnant women or in women of child-bearing potential if the potential benefit justifies the potential risk to the foetus; it is considered that untreated tuberculosis represents a far greater hazard to a pregnant woman and her foetus than treatment of the disease. Pyridoxine supplementation is recommended. Breast-feeding: isoniazid passes into breast milk — monitor breast-fed infants for possible signs of isoniazid toxicity, and administration of pyridoxine to the breast-feeding mother and infant may be considered.

Isoniazid

Brand names: Rimifon

Isoniazid is a first-line bactericidal antimycobacterial agent used in combination for treating active tuberculosis and alone for the treatment of latent tuberculosis infection.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 4 to 5 mg per kilogram body-weight daily
Route: Oral — take preferably on an empty stomach, at least 30 minutes before a meal or 2 hours after a meal; tablets must be swallowed whole and not chewed
Frequency: Daily, in single or divided doses
Max: 300 mg daily
Official guidance should always be consulted when selecting the dose regimen for adults and children (according to age and body weight), the duration of therapy and the total content of the combination treatment regimen. HIGHER DOSE IN TUBERCULOUS MENINGITIS: up to 10 mg per kilogram body-weight daily may be given, particularly during the first 1 to 2 weeks of treatment of tuberculous meningitis. INTERMITTENT REGIMEN (a separate regimen — do NOT combine with the daily maximum above): a dose of 15 mg per kilogram body-weight has been given two or three times weekly in intermittent treatment regimens. ELDERLY: no dosage reduction is necessary, but exercise caution because of the possible decrease in renal and hepatic function. NEUROPATHY PROPHYLAXIS: patients at risk of neuropathy or pyridoxine deficiency — including those who are diabetic, alcoholic, malnourished, uraemic, pregnant or infected with HIV — should be given pyridoxine (the SPC does not state a pyridoxine dose). MONITORING: all patients should have baseline liver function tests, repeated at regular intervals during treatment; withdraw treatment if serum AST rises to more than three times normal or if there is any increase in bilirubin. Special precautions are required in patients with impaired liver function, and any deterioration in liver function in these patients is an indication for stopping treatment. Discontinue immediately if symptoms of hepatitis such as malaise, fatigue, anorexia and nausea develop. Take care in patients with convulsive disorders, diabetes mellitus, chronic alcoholism, impaired liver or kidney function, or taking other potentially hepatotoxic agents; use with caution in patients with a history of psychosis. Risk factors identified for isoniazid-induced hepatotoxicity are advanced age, female gender, slow acetylator status, malnutrition, HIV infection, pre-existing liver disease and extra-pulmonary tuberculosis. WITHDRAWAL: symptoms on cessation may include headache, insomnia, excessive dreaming, irritability and nervousness. NOTE ON COMPLETENESS: the fetched section 4.5 is truncated partway through the pyridoxine entry, so the interaction list below is not exhaustive — check the full SPC.

Paediatric dose

Dose: 10 mg/kg
Route: Oral — preferably on an empty stomach, at least 30 minutes before a meal or 2 hours after a meal
Frequency: Daily, in single or divided doses
Max: No separate paediatric ceiling dose is stated in this SPC — verify the total daily dose against a children's formulary before prescribing
The SPC states the usual daily dose for children aged three months and above is from 10 UP TO 15 mg per kilogram body-weight daily in single or divided doses — 10 mg/kg is the lower bound of that range and is recorded here as the conservative per-kg figure; the full 10-15 mg/kg range must be read before prescribing. Isoniazid should NOT be used in children aged 0 to 3 months because of the lack of specific data. The SPC gives no intermittent (two or three times weekly) paediatric regimen — do not apply this daily per-kg figure to an intermittent schedule. For reference, the stated adult maximum is 300 mg daily. Official guidance should always be consulted when selecting the paediatric dose by age and body weight, the duration of therapy and the combination regimen. Verify this dose against a children's formulary and specialist paediatric infectious disease advice.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

The SPC states the usual daily dose for children aged three months and above is from 10 UP TO 15 mg per kilogram body-weight daily in single or divided doses — 10 mg/kg is the lower bound of that range and is recorded here as the conservative per-kg figure; the full 10-15 mg/kg range must be read before prescribing. Isoniazid should NOT be used in children aged 0 to 3 months because of the lack of specific data. The SPC gives no intermittent (two or three times weekly) paediatric regimen — do not apply this daily per-kg figure to an intermittent schedule. For reference, the stated adult maximum is 300 mg daily. Official guidance should always be consulted when selecting the paediatric dose by age and body weight, the duration of therapy and the combination regimen. Verify this dose against a children's formulary and specialist paediatric infectious disease advice.

Verify in a children's formulary

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients
  • Previous experience of severe adverse reaction to isoniazid, including drug-induced liver disease

Side effects

  • Peripheral neuropathy, seizure and optic neuritis; hyperreflexia may be troublesome with doses of 10 mg per kg body weight
  • Hepatobiliary: hepatitis (uncommon), acute hepatic failure, liver injury, jaundice and increased hepatic enzymes — risk increases with age, especially over 35 years, and may be serious and sometimes fatal with the development of necrosis
  • Hypersensitivity reactions including various types of skin eruptions, fever and lymphadenopathy; erythema multiforme and Stevens-Johnson syndrome; toxic epidermal necrolysis and DRESS (rare)
  • Psychotic disorder and euphoria; mental disturbances ranging from minor personality changes to major mental derangement, usually reversed on withdrawal
  • Haemolytic and aplastic anaemia, agranulocytosis; nausea, vomiting, constipation, dry mouth and pancreatitis; hyperglycaemia, hypoglycaemia, metabolic acidosis and pellagra; deafness, tinnitus and vertigo (reported in end stage renal impairment; vertigo may be troublesome with doses of 10 mg per kg body weight)

Interactions

  • Para-aminosalicylic acid, and slow acetylator status — tissue concentrations of isoniazid may be enhanced and adverse effects are more likely
  • Rifampicin — possible increased risk of liver damage, although liver enzymes are raised only transiently; concurrent use of any other hepatotoxic medication may increase the potential for hepatotoxicity
  • Carbamazepine — isoniazid can cause substantial elevations of serum carbamazepine and symptoms of carbamazepine toxicity at isoniazid doses of 200 mg daily or more; concurrent use is not recommended unless effects can be closely monitored and the carbamazepine dose reduced (a reduction of one-half to one-third was reported effective)
  • Phenytoin and primidone — isoniazid inhibits hepatic metabolism; phenytoin dosage adjustment may be necessary during and after isoniazid therapy, especially in slow acetylators
  • Benzodiazepines (diazepam, triazolam) — increased risk of benzodiazepine toxicity (sedation, respiratory depression). Also chlorzoxazone and disulfiram. Isoniazid may increase renal excretion of pyridoxine, raising pyridoxine requirements (section 4.5 truncated at this point in the fetched source)

Clinical monograph

How it works

It is a prodrug activated by mycobacterial catalase-peroxidase (KatG) that inhibits mycolic acid synthesis, an essential component of the mycobacterial cell wall.

Prescribing in practice

  • Hepatotoxicity is the most serious risk; advise patients to stop and seek review if symptoms of liver injury occur, and assess baseline liver function.
  • Pyridoxine is co-prescribed to prevent peripheral neuropathy, particularly in those at higher risk such as diabetes, alcohol dependence, malnutrition, HIV or pregnancy.
  • It is a potent enzyme inhibitor and can raise levels of interacting drugs such as phenytoin and carbamazepine.

Monitoring

Check liver function before treatment and monitor clinically (with repeat testing if abnormal at baseline or if symptoms develop) throughout therapy.

Counselling the patient

  • Stop the medicine and seek urgent advice if you develop nausea, vomiting, jaundice or dark urine.
  • Report numbness or tingling in the hands or feet, which the vitamin B6 tablet helps prevent.
  • Avoid alcohol and take the full course to cure the infection and prevent resistance.

Evidence & guidelines

Isoniazid is a core component of standard antituberculosis regimens recommended by NICE (NG33) and WHO.

Reference: NICE NG33 TB; British Thoracic Society TB Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

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Curated clinical cross-links plus same-class fallbacks.

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