Isoniazid: What You Need to Know Before You Start

If your doctor prescribed Isoniazid, you’re probably dealing with tuberculosis (TB) or trying to prevent it after exposure. Isoniazid is an old‑school antibiotic that still works great against the TB bacteria. It’s taken by mouth, usually once a day, and can be part of a longer combo therapy or a solo preventive plan.

How Isoniazid Works and When It’s Used

In simple terms, Isoniazid stops the TB bug from building its cell wall, which kills the bacteria over time. Doctors use it for two main reasons: treating active TB along with other drugs, and preventing TB in people who have been exposed but aren’t sick yet. The preventive version is called “latent TB treatment” and often lasts 6 to 9 months.

Because Isoniazid is taken for weeks or months, sticking to the schedule matters. Missing doses can let the bacteria bounce back and may lead to resistance.

Dosage, Side Effects, and Safety Tips

Typical adult doses are 300 mg once daily for treatment, or 300 mg weekly for prevention, adjusted for weight and liver health. Kids get a dose based on weight (usually 10–15 mg/kg). Your doctor will check liver enzymes before you start and may repeat the test during therapy, especially if you’re over 35 or have liver disease.

Common side effects are mild: nausea, loss of appetite, and a metallic taste. Some people notice numbness or tingling in the hands and feet—this is a sign of vitamin B6 (pyridoxine) deficiency. Doctors often add a small B6 supplement to avoid that problem.

Serious but rare reactions include liver injury, which can show up as yellow skin, dark urine, or severe fatigue. If any of those happen, call your doctor right away. Alcohol can increase liver risk, so limit drinks while on Isoniazid.

Drug interactions matter, too. Isoniazid can raise the levels of certain meds like phenytoin, carbamazepine, and some HIV drugs, making side effects worse. Tell your prescriber about every prescription, over‑the‑counter, and herbal product you take.

To keep things safe, take Isoniazid on an empty stomach—usually an hour before or two hours after food. Swallow the tablet whole with a glass of water; crushing or chewing can irritate your throat.

Finally, remember that Isoniazid is not a cure‑all for TB. It works best when paired with other TB drugs for active infections, and the full course must be finished even if you start feeling better.

Need more details? Browse our other articles on TB drugs, liver health, and vitamin B6 supplementation to round out your knowledge.