isoniazid at a glance
Medicine class
Antituberculosis antibacterial
Licensed uses
TB affecting the lungs or other parts of the body
Food
Take on an empty stomach; food can reduce absorption
Key monitoring
Liver function and symptoms of nerve damage
What is isoniazid used for?
The selected tablet is licensed for:
Pulmonary tuberculosis.
Extrapulmonary tuberculosis, affecting sites outside the lungs.
Specialists may also use isoniazid outside the selected product's stated indication to prevent active disease in selected people with latent TB infection.
How does isoniazid work?
Isoniazid is activated inside Mycobacterium tuberculosis. Its active form inhibits InhA, disrupting production of mycolic acids needed for the bacterial cell wall and impairing or killing susceptible bacteria.
isoniazid preparations and strengths
Tablet
Route: Oral
Strengths: 50 mg, 100 mg
How to take isoniazid
Follow the prescription and TB team's instructions. Swallow the tablets whole with water on an empty stomach.
Avoid alcohol because it can increase the risk of liver injury.
Certain matured cheeses, cured meats, alcoholic drinks and some fish can cause headache, flushing, sweating, palpitations or dizziness; ask the TB team about foods to avoid.
Do not stop treatment without speaking to the TB team.
isoniazid side effects
Common or expected effects
- Nausea, vomiting or other stomach upset; frequency is not known.
- Tingling or numbness in the hands or feet, particularly in people at risk of vitamin B6 deficiency.
- Dizziness, dry mouth, constipation, fever or rash; frequencies are not known.
Get urgent medical advice
- Liver inflammation or failure.
- Severe skin reactions with a spreading rash, blisters, peeling skin, mouth sores or fever.
- Seizures, hallucinations, delusions or major behavioural changes.
- Severe abdominal and back pain suggesting pancreatitis.
- Visual disturbance or eye pain, severe blood disorders, vasculitis or interstitial lung disease.
isoniazid in pregnancy
Isoniazid crosses the placenta. Pregnancy does not automatically prevent treatment: untreated active TB can present a greater risk, but use should be decided with the specialist TB and maternity teams. Pyridoxine supplementation is recommended when isoniazid is used during pregnancy.
isoniazid while breastfeeding
Isoniazid passes into breast milk, but UK public-health information supports breastfeeding during first-line TB treatment because milk concentrations are low. The infant should be observed for possible toxicity, and the specialist team should consider pyridoxine for the mother and infant. Breast-milk exposure does not treat or prevent TB in the baby.
isoniazid interactions
Tell the TB team about all prescribed, pharmacy and herbal products. Important interactions include:
Carbamazepine, phenytoin and primidone
Isoniazid can increase anticonvulsant toxicity, so clinical or concentration monitoring may be needed.
Diazepam and triazolam
Sedation and respiratory depression may be increased.
Disulfiram
Combined use can increase adverse effects and requires specialist review.
Rifampicin and other potentially hepatotoxic medicines
The risk of liver injury may increase, requiring careful monitoring.
Common questions about isoniazid
Answers are fully visible for fast scanning and source review.
Why is vitamin B6 given with isoniazid?
Isoniazid can contribute to vitamin B6 deficiency and peripheral neuropathy. Pyridoxine is particularly important for people with recognised risk factors.
What should I do about pins and needles?
Contact the TB team promptly. Tingling or numbness may indicate peripheral neuropathy and should be assessed.
Why are blood tests needed?
Sources
These sources were used to prepare and review this medicine page.
Prepared and reviewed by the iatroX editorial team.