Isoniazid: Uses, Interactions and Safety

Uses

Isoniazid is a key medicine for tuberculosis. Active TB requires an effective combination selected according to drug susceptibility; isoniazid must not be used alone for active disease. It is also used in selected preventive regimens after active TB has been excluded. Treatment and follow-up are organised by the TB team.

Administration

This page covers oral tablets. They are generally taken on an empty stomach according to the product instructions. The team may prescribe pyridoxine to reduce neuropathy risk, especially during pregnancy or with diabetes, malnutrition or other risk factors. Do not omit other TB medicines or stop the regimen simply because symptoms improve.

Safety

Isoniazid can cause serious hepatitis. Baseline assessment and symptom review are important, with blood tests according to risk. Stop taking it and contact the team immediately if persistent nausea, loss of appetite, marked tiredness, dark urine or jaundice develops. Acute liver disease and previous isoniazid-related liver injury are important contraindications.

Interactions

Phenytoin, carbamazepine, theophylline and some other medicines may accumulate and require monitoring or adjustment. Diazepam effects can increase. Itraconazole may become less effective. Rifampicin and pyrazinamide are often intentional partners, but combined liver toxicity needs monitoring. Avoid alcohol; tyramine- or histamine-rich foods can also cause troublesome reactions.

Pregnancy and breastfeeding

Isoniazid can be used for necessary TB treatment during pregnancy under specialist care. Untreated TB also threatens the pregnancy. Breastfeeding requires consideration of maternal and infant monitoring and pyridoxine. Milk exposure does not treat or prevent TB in the infant.

Adverse effects

Nausea, vomiting and stomach discomfort are reported. Tingling, numbness, burning pain or weakness in the hands and feet may signal neuropathy. Severe rash, seizures or altered consciousness require urgent assessment. Report adverse effects promptly so the team can protect both safety and the effectiveness of the complete TB regimen.

Sources

• Isoniazid tablets: US prescribing information
• Isoniazid 100 mg Tablets: Summary of Product Characteristics
• Turkish INH tablet product information
• Turkish Ministry of Health tuberculosis diagnosis and treatment guideline (2026)
• WHO ATC/DDD Index 2026: J04AC01
• Pyrazinamide 150 mg dispersible tablets, TB307: WHOPAR Part 4