Isoniazid

SCOPE: this draft is distilled from the same UK SPC (Isoniazid 100 mg Tablets) as the id 'isoniazid' — the SPC gives a single tuberculosis posology and contains no respiratory-specific or latent-TB-specific regimen, so no separate respiratory dosing has been invented. The SPC states that 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.

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 respiratory or infectious disease advice.