isoniazid: clinical use

Prescribing considerations

• Review liver disease, alcohol use, renal impairment, seizures, diabetes, malnutrition, HIV and previous psychosis.
• Give pyridoxine to patients at increased risk of neuropathy or deficiency, including those who are pregnant, malnourished, diabetic, alcohol-dependent, uraemic or living with HIV.
• Counsel patients about hepatitis, neuropathy and severe cutaneous reactions.

Contraindications and cautions

• Hypersensitivity to isoniazid.
• Previous severe adverse reaction to isoniazid, including drug-induced liver disease.
• Do not restart after isoniazid-associated Stevens–Johnson syndrome, toxic epidermal necrolysis, DRESS or acute generalised exanthematous pustulosis.
• The selected lactose-containing product is unsuitable in specified rare hereditary disorders of lactose or galactose handling.

Monitoring

• Obtain baseline liver-function tests and repeat regularly.
• Withdraw treatment if AST exceeds three times normal or bilirubin increases, as specified by the selected product information.
• Monitor more closely when hepatic risk factors or potentially hepatotoxic co-medication are present.
• Assess new paraesthesia, visual symptoms, psychiatric changes, seizures or severe rash promptly.
• Review interacting medicines and consider therapeutic drug monitoring where appropriate.

Clinical pharmacology

Isoniazid is readily absorbed orally, distributes widely including into cerebrospinal fluid, crosses the placenta and enters breast milk. Hepatic acetylation is genetically polymorphic, producing rapid and slow acetylator phenotypes that influence exposure and toxicity.

Formulation and product differences

• The selected product is a white to off-white, biconvex, uncoated tablet.
• It contains lactose monohydrate and is intended for oral administration.
• Not all authorised pack sizes may be marketed.

Tablet

Route: Oral

Strengths: 50 mg, 100 mg