Isoniazid Dosage: What You Need to Know

If you or someone you know has been prescribed isoniazid, the biggest question is usually "how much should I take?" Isoniazid is a key drug for treating tuberculosis (TB) and the right dose makes the difference between cure and trouble.

Most doctors start with a standard adult dose of 5 mg per kilogram of body weight, up to a maximum of 300 mg per day. That means a 70 kg (154 lb) adult will usually get 300 mg once daily. The medicine is taken by mouth, with or without food – just try to keep the timing the same each day.

Special Situations: Kids, Liver Problems, and Pregnancy

Kids get a lower dose: about 10 mg per kilogram, but never more than 300 mg daily. For a child weighing 20 kg, that works out to 200 mg a day, usually split into one or two doses.

If you have liver disease, the dose may need to be cut. The liver processes isoniazid, so a reduced amount (often 2–3 mg/kg) helps avoid damage. Your doctor will check liver enzymes before starting and monitor them during treatment.

Pregnant women can usually stay on the same dose as non‑pregnant adults. Isoniazid is considered safe in pregnancy, but your doctor will watch both you and the baby closely.

How Long to Take It and Why Combination Matters

For most active TB cases, isoniazid is taken for six months as part of a combination regimen (often with rifampicin, pyrazinamide, and ethambutol). Sticking to the full course is critical – stopping early can let the bacteria become resistant.

In some cases, doctors prescribe isoniazid alone for nine months to prevent latent TB from becoming active. The daily dose stays the same, but the length of therapy changes.

Never change the dose on your own. If you miss a dose, take it as soon as you remember unless it’s almost time for the next one – then just skip the missed one and continue as usual. Double‑dosing can raise the risk of side effects.

Common side effects include mild nausea, loss of appetite, or a temporary loss of taste. More serious issues like liver injury are rare but need prompt medical attention. Watch for dark urine, yellow skin, or severe fatigue, and call your doctor right away.

To keep the drug working, store it at room temperature, away from moisture and heat. Keep the bottle tightly closed and out of reach of children.Bottom line: the usual adult dose is 300 mg once daily, kids get about 10 mg/kg, and adjustments are made for liver problems or other special conditions. Follow your doctor’s instructions, finish the whole course, and report any odd symptoms immediately. With the right dosage, isoniazid is a powerful tool to beat TB.