Isoniazid

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.