Isoniazid: What It Is, How to Use It, and What to Watch For

Isoniazid is a medicine doctors use to treat and prevent tuberculosis (TB). It stops the TB bacteria from making the cell wall they need to survive. Because TB can hide in the lungs for years, a short‑term cure rarely works; most patients stay on isoniazid for six months or longer.

How Isoniazid Works and Who Needs It

When you take isoniazid, it gets turned into a toxic form inside the bacteria. That toxic form blocks an enzyme called InhA, which is essential for building the bacterial cell wall. Without a strong wall, the bacteria burst and die. The drug works best on bacteria that are actively growing, so it’s often paired with other TB medicines during the intensive phase of treatment.

Doctors prescribe isoniazid for three main reasons:

• Active TB infection – as part of a combination regimen.
• Latent TB infection – when the bacteria are present but not causing disease.
• Pre‑exposure prevention – for people at high risk, such as close contacts of an active case.

If you’ve been told you have a positive TB skin test or interferon‑gamma release assay (IGRA) but no symptoms, your doctor may suggest a 6‑month isoniazid course to stop the infection from turning active.

Dosage, Side Effects, and Safety Tips

The usual adult dose is 5 mg per kilogram of body weight, taken once a day. Most tablets are 100 mg, so a typical adult ends up taking 300 mg daily. Kids get a similar weight‑based dose, but always follow the paediatric prescribing chart.

Take isoniazid on an empty stomach – at least one hour before or two hours after food – because food can lower how much gets into your bloodstream. A glass of water with the tablet helps avoid stomach irritation.

Common side effects include mild nausea, headache, and a metallic taste. These often fade after a few days. More serious issues to watch for are:

• Liver problems – yellow skin or eyes, dark urine, severe fatigue.
• Peripheral neuropathy – tingling or numbness in the hands and feet.
• Allergic reactions – rash, itching, swelling.

Liver toxicity occurs in about 1‑2 % of patients, especially those over 35, heavy drinkers, or people with pre‑existing liver disease. Your doctor will likely order liver function tests at baseline and then every couple of months.

Peripheral neuropathy happens because isoniazid can deplete vitamin B6 (pyridoxine). To prevent this, many prescribers add a daily 10‑25 mg vitamin B6 supplement, especially for pregnant women, diabetics, or anyone taking the drug for more than three months.

Never stop the medication on your own, even if you feel better. Stopping early can let hidden bacteria become resistant, making future TB harder to treat. If you miss a dose, take it as soon as you remember unless it’s almost time for the next one – then skip the missed dose.

Drug interactions matter. Isoniazid can increase blood levels of certain drugs such as phenytoin, carbamazepine, and some HIV meds. Always tell your pharmacist about every prescription and over‑the‑counter product you use.

Pregnant or breastfeeding people should discuss risks with their doctor. Isoniazid crosses the placenta but is generally considered safe when the benefits outweigh the risks. Vitamin B6 supplementation is especially important during pregnancy.

In summary, isoniazid is a key part of TB control. Taking it exactly as prescribed, watching for liver signs, adding vitamin B6, and keeping up with lab checks will help you finish the course safely. If you have any new symptoms, call your healthcare provider right away.