Isoniazid for Tuberculosis

Isoniazid is an antibacterial drug used for all forms of tuberculosis in which the germs are not resistant to it. It is one of the four RIPE drugs and is also used to treat inactive TB. INH is its common abbreviation¹. In CDC regimen names from 2025, such as 3HP and 9H, the letter H stands for isoniazid³. The label gives its chemical name as isonicotinic acid hydrazide, with the formula C6H7N3O¹. The pure drug is a white crystalline powder that dissolves freely in water¹. For active TB, isoniazid alone is not enough, so it is always combined with other TB drugs². StatPearls (2024) calls it a first-line drug that has been central to TB treatment for decades⁶.

Like every TB drug, it has side effects worth knowing before you start. The guide to isoniazid side effects lists them with what to do about each.

Isoniazid works by stopping TB bacteria from making mycolic acids, which their cell wall needs. The drug label calls mycolic acids an essential part of the bacterial cell wall. StatPearls (2024) describes it as a prodrug, switched on inside the bacterium by an enzyme called KatG⁶. At treatment levels it kills actively growing TB bacteria¹. Blood levels peak 1 to 2 hours after a dose, and the drug reaches all body fluids and tissues¹. The body breaks it down by acetylation, at a speed set by your genes¹. About half of Black and white people are slow acetylators¹. Slow acetylators may have higher blood levels and more side effects¹. The speed does not much change how well it works¹. Within 24 hours, 50 to 70 percent of a dose leaves in the urine¹.

Isoniazid is part of most US TB regimens, for both active and inactive TB. For active TB it is in the 6-month RIPE regimen and the 4-month rifapentine-moxifloxacin regimen.

CDC guidance from 2025 prefers 3HP and 3HR to isoniazid alone, which has more side effects and lower completion³. In RIPE, all four drugs are taken for 2 months, then isoniazid and rifampin alone for 4 more months². The 4-month regimen is for lung TB not thought to be drug-resistant². It suits people 12 or older who weigh at least 40 kg². CDC guidance from 2025 uses a 9-month version, with 7 months of isoniazid and rifampin². It applies when lung cavities show and the 2-month culture is still positive². 6H is strongly recommended for HIV-negative people of any age, and 9H is an option with or without HIV³. The CDC regimens update from 2025 explains how these choices are made³.

Yes: isoniazid treats inactive TB, the infection without symptoms, so that it does not become TB disease. CDC guidance from 2025 rules out TB disease first and gives priority to people with risk factors. The label recommends preventive treatment for these 7 groups¹.

• People with HIV
• Close contacts of someone with infectious TB
• People whose skin test turned positive in the last 2 years
• People with scars of old healed TB on a chest X-ray
• People who inject drugs
• People with silicosis, diabetes or end-stage kidney disease
• People on long-term steroids or other immune-suppressing treatment

Over age 35, the label weighs the risk of hepatitis against the risk of TB¹. People without known risk factors may still be considered, CDC said in 2025³. That follows a positive blood test or a skin test of 15 mm or more³. Treatment keeps latent tuberculosis (TB) infection, which is not contagious, from turning into disease.

In most US regimens, isoniazid is taken for 3 to 9 months, depending on the plan and the type of TB. CDC guidance from 2025 gives 4, 6 or 9 months for active TB and 3, 4, 6 or 9 for inactive TB.

Stopping early raises the chance that TB comes back⁵. Missing too many doses raises the chance of drug-resistant TB⁵. The label gives children 12 months of treatment for TB meningitis, bone and joint TB, and miliary TB¹. When you finish, ask for a written record, because later TB tests may stay positive⁴.

The usual isoniazid dose for adults is 5 mg per kilogram once a day, up to 300 mg. To prevent TB, adults over 30 kg take 300 mg once a day. Some regimens give 15 mg per kilogram, up to 900 mg, two or three times a week¹. It comes as 100 mg and 300 mg tablets¹. Food lowers how much is absorbed, so it is taken without food¹. In the weekly regimen, adults take 15 mg per kilogram once a week, up to 900 mg, with rifapentine (Priftin) 3HP³. The Teva tablets are white and scored, round at 100 mg and oval at 300 mg¹.

You should take isoniazid as one single dose on each treatment day, whether your plan is daily, weekly or twice a week. With directly observed therapy, a health worker watches you take each dose, in person or by video. Three habits keep treatment on track⁵.

1. If you miss a dose, skip it and take the next scheduled dose.
2. Tell your provider about the missed dose.
3. Store the tablets at 68 to 77°F, away from moisture and light.

CDC guidance from 2025 calls directly observed therapy for tuberculosis the standard of care². It is used whenever doses are taken less than 7 days a week².

Vitamin B6, also called pyridoxine, is taken with isoniazid to prevent nerve damage in the hands and feet. That nerve damage, felt as numbness or tingling, is the most common toxic effect of isoniazid. The label recommends B6 for people who are malnourished or prone to nerve damage, such as people with diabetes or alcoholism¹. CDC noted in 2025 that providers may add it⁴. CDC guidance from 2025 gives 25 to 50 mg of B6 a day to everyone at risk of nerve damage². That group includes people with HIV, chronic kidney failure or older age, and breastfed infants². Experts cited by CDC in 2025 advise 100 mg a day for nerve damage already present². CDC advised in 2025 stopping the medicine and seeking care for lasting nerve symptoms⁴. These are numbness, pain, tingling, or hot or cold feelings in the hands or feet⁴. A balanced tuberculosis diet supports the body during months of treatment.

While taking isoniazid, you should avoid foods high in tyramine or histamine, the drug label says. Cheese and red wine are high in tyramine, and tuna, skipjack and other tropical fish are high in histamine. Histamine foods can cause headache, flushing or low blood pressure with isoniazid¹. Daily alcohol raises the risk of liver injury¹. Medicines matter too, as the page on isoniazid drug interactions explains.

The boxed warning for isoniazid is about severe and sometimes fatal hepatitis, a liver injury. The risk rises with age, from about 1 in 1,000 under age 20 to 23 in 1,000 at 50 to 64. These 6 warning signs should be reported at once¹.

• Loss of appetite
• Nausea or vomiting
• Dark urine
• Yellow skin or eyes
• Lasting tiredness or fever over 3 days
• Tenderness in the upper right belly

People 35 and older have liver tests before and during treatment, and everyone is seen monthly¹. The risk is about 3 in 1,000 at 20 to 34 and 12 in 1,000 at 35 to 49¹. It is about 8 in 1,000 at 65 and older¹. Daily alcohol, chronic liver disease and injection drug use raise it further¹. Women of minority groups, especially Black and Hispanic women, and women who have just given birth need closer watching¹.

Isoniazid needs regular checks for side effects and TB symptoms, and some people also need liver blood tests. CDC guidance from 2025 does not make liver tests routine at the start, but advises them for some groups. Those groups include people with a liver disorder, regular alcohol or injection drug use, or HIV³. Pregnancy or a birth in the last 3 months is on the list too³. Isoniazid is held if liver enzymes pass 3 times the upper normal limit with symptoms, or 5 times without³.

An isoniazid overdose causes symptoms within 30 minutes to 3 hours, and the antidote is pyridoxine, vitamin B6, given into a vein. Early signs are nausea, vomiting, dizziness, slurred speech, blurred vision and seeing things that are not there. A severe overdose leads to breathing trouble, seizures that are hard to stop, and coma¹. Pyridoxine is given gram for gram, equal to the isoniazid dose taken¹. If the amount is unknown, the label suggests 5 grams for adults and 80 mg per kilogram for children¹.

People who had liver injury or another severe reaction from isoniazid before should not take it again. The label also rules it out for anyone with acute liver disease of any cause. Severe reactions named on the label include drug fever, chills and arthritis¹.

These questions cover its name, its drug class, children and pregnancy.