Isoniazid

Isoniazid is one of the main medicines used to treat tuberculosis (TB), together with other anti-TB medicines, and to prevent TB in people at risk. This guide covers isoniazid uses, how it works, dosage, side effects, precautions, drug interactions, use in pregnancy and breastfeeding and its approximate price in India.

At a Glance

What is Isoniazid?

Isoniazid is one of the main medicines used to treat tuberculosis. For active TB disease, it is used together with other anti-TB medicines, and not on its own. It is also used as part of TB preventive treatment for people who have TB infection without active disease (latent TB infection) or who are at high risk of developing TB. Isoniazid should be taken only under a doctor’s supervision.

How Isoniazid Works

Isoniazid inhibits the synthesis of mycolic acid, which disrupts the bacterial cell wall. At therapeutic levels it is bactericidal against most strains of Mycobacterium tuberculosis.

When Isoniazid Is Used

• Active TB: Isoniazid is used in combination with other anti-TB medicines for TB caused by susceptible organisms. Combination treatment is essential to cure the disease and reduce the risk of drug resistance.
• TB preventive treatment (latent TB infection): Isoniazid is part of several preventive regimens for people at risk of developing TB disease, such as people with HIV infection, close contacts of someone with active TB, and people with recent evidence of TB infection. Active TB must be excluded before preventive treatment is started. Your doctor decides who needs it and which regimen is suitable.

Isoniazid Dosage and How to Take It

The dose depends on whether isoniazid is used for treatment or prevention, your age and weight, the regimen and your response. The examples below come from US prescribing information and CDC, WHO and Indian National TB Elimination Programme (NTEP) guidance, as indicated. They are general information only. Your doctor decides the regimen and dose.

Active TB (US label and CDC)

• Adults: 5 mg/kg up to 300 mg once daily as part of a multidrug regimen.
• Children: 10 to 15 mg/kg up to 300 mg once daily, according to the US label. CDC lists 10 to 20 mg/kg (maximum 300 mg) for children, and the American Academy of Pediatrics recommends 10 to 15 mg/kg for the daily regimen, so a child’s dose is set by the doctor according to age and weight.
• Regimen example (CDC): The standard regimen for drug-susceptible TB combines isoniazid, rifampin, pyrazinamide and ethambutol for 2 months (8 weeks), followed by isoniazid and rifampin for 4 months (18 weeks), about 6 months in total. Treatment may be extended, for example to 9 months, in some situations, and drug-resistant TB needs different regimens. In India, drug-sensitive TB is treated under the NTEP with a standard daily multidrug regimen prescribed according to your weight, so follow the regimen given by your treating doctor or TB programme provider.
• Intermittent dosing: The US label also lists schedules of two or three times a week (15 mg/kg, up to 900 mg per dose). These are not routine, daily dosing is generally preferred in current guidance, and intermittent schedules are used only in supervised settings. 900 mg is not a daily dose.

Preventive treatment (CDC, WHO and NTEP)

• Regimens that contain isoniazid: Daily isoniazid alone for 6 months (6H) or 9 months (9H), weekly isoniazid plus rifapentine for 3 months (3HP), and daily isoniazid plus rifampin for 3 months (3HR). WHO also lists a 1-month daily isoniazid plus rifapentine regimen (1HP) as an alternative. The shorter regimens combine isoniazid with another medicine, so they are not isoniazid alone. A 4-month regimen of daily rifampin alone (4R) does not contain isoniazid.
• Which regimen: CDC preferentially recommends the 3- or 4-month rifamycin-based regimens over 6 or 9 months of isoniazid alone, and keeps 6H and 9H as alternatives, for example when drug interactions rule out rifamycins. WHO recommends all of these options regardless of HIV status. India’s NTEP lists 6H and 3HP (with 3RH and 1HP in addenda), and for pregnant and breastfeeding people living with HIV and for children under 13 years the preferred NACO/NTEP regimen is daily isoniazid for 6 months. The choice depends on your age, health, other medicines and the guidance followed.
• Duration is not universal: Some US isoniazid labels say people with HIV should have at least 12 months of isoniazid. Current CDC, WHO and Indian guidance instead recommends the regimens above, so the length of preventive treatment depends on the selected regimen and your circumstances.
• Isoniazid-alone dose: Adults 5 mg/kg (maximum 300 mg) daily per CDC. Children are dosed by age and weight, for example 10 mg/kg per day for children under 10 years under NTEP guidance, so the doctor sets the dose.

Taking it: Take isoniazid exactly as prescribed, at the same time each day, and complete the full course even when you feel better.

Take this medicine only as advised by your doctor or healthcare professional. Do not start, stop, or change the dose on your own.

Side Effects of Isoniazid

Common effects may include:

• Numbness, tingling or burning in the hands and feet (peripheral neuropathy), which is the most common toxic effect
• Nausea, vomiting and stomach upset
• Skin rash
• Fever
• Tiredness

Serious effects need prompt medical attention:

• Hepatitis (liver inflammation): loss of appetite, nausea or vomiting, persistent tiredness or weakness, dark urine, abdominal pain or discomfort, or yellowing of the skin or eyes. This can be severe and sometimes fatal and can develop even after many months of treatment.
• Severe allergic and skin reactions: swelling of the face or throat, breathing difficulty, widespread rash, blistering or peeling skin, or fever with rash
• Nerve and eye problems: blurred vision or eye pain (optic neuritis), or worsening numbness or weakness
• Blood problems: unusual bruising or bleeding, or fever and sore throat
• Mood and thinking changes, such as confusion

Isoniazid Precautions and Monitoring

• Liver safety and monitoring: Everyone taking isoniazid should be assessed clinically, usually at least monthly, for symptoms of liver injury, and should be taught the warning signs. Doctors may also check liver function (AST/ALT, bilirubin) at the start and during treatment, especially for people with risk factors such as age over 35, HIV infection, chronic liver disease, regular alcohol use, or pregnancy or the postpartum period, and for anyone with symptoms. Blood tests are not a substitute for a clinical check, and a clinical check alone is not enough for people at higher risk.
• If you suspect liver injury: Contact your doctor or TB clinic immediately. Do not carry on regardless, and do not stop or restart TB medicines on your own for the long term. Your doctor will assess you promptly and decide whether to pause or change treatment.
• Contraindications (when isoniazid should not be used): previous severe hypersensitivity reactions to isoniazid, including drug-induced hepatitis, previous isoniazid-associated liver injury, severe reactions such as drug fever, chills or arthritis, and acute liver disease of any cause.
• Extra caution and closer monitoring (not absolute contraindications): older age (especially over 35), daily alcohol use, chronic liver disease, injection drug use, severe kidney problems, HIV infection, existing peripheral neuropathy or conditions that make it more likely (such as diabetes or poor nutrition), and pregnancy or the postpartum period. Your doctor decides whether and how to use isoniazid in these situations.
• Vitamin B6 (pyridoxine): Pyridoxine can prevent and reverse peripheral neuropathy caused by isoniazid. WHO advises vitamin B6 supplements for people at risk of neuropathy, such as those with malnutrition, HIV, kidney failure or diabetes, or who are pregnant or breastfeeding. It is not required for everyone, so take it only as advised by your doctor.
• Alcohol: Daily alcohol use increases the risk of hepatitis, so avoid alcohol while taking isoniazid.
• Stopping treatment: Missing doses or stopping early can allow the TB to come back and can lead to drug-resistant TB. Do not stop on your own, even if you feel better.

Isoniazid Drug Interactions

Interactions can be important, so always check with your doctor or pharmacist. Your doctor will decide whether a combination is appropriate and how closely to monitor you.

• Carbamazepine and phenytoin: Isoniazid can raise their blood levels, which may increase side effects, so your doctor may monitor you.
• Disulfiram: Taking it with isoniazid can cause behavior and coordination problems, so the combination should be avoided.
• Aluminum-containing antacids: can reduce the absorption of isoniazid. Ask your doctor or pharmacist how to space them.
• Rifampin and other anti-TB medicines: These are routinely used with isoniazid under supervision, and the combination can add to liver effects, which is why monitoring is important.
• Alcohol: Increases the risk of liver injury.
• Foods very high in tyramine or histamine (such as some aged cheeses and certain fish): Some labels advise avoiding them, because reactions such as flushing, headache or palpitations have been reported with isoniazid. Not everyone is affected, and the risk differs by food, so ask your doctor or dietitian if you are unsure.

Tell your doctor and pharmacist about all prescription medicines, over-the-counter medicines, supplements and herbal products you take. This list is not complete.

Isoniazid in Pregnancy and Breastfeeding

1. Pregnancy: Treatment decisions should be made with your treating clinician. Isoniazid crosses the placenta, and US labeling advises observing newborns of treated mothers for any effects. Isoniazid is not automatically prohibited in pregnancy. India’s NTEP lists the 6-month isoniazid regimen (6H) as safe for use in pregnancy, while data for 3HP are not known. Untreated TB is also dangerous for mother and baby, and pregnant and postpartum women may need closer liver monitoring, so do not stop prescribed TB treatment on your own.
2. Breastfeeding: Isoniazid passes into breast milk in small amounts. US labeling states that these small amounts do not produce toxicity in the nursing newborn, so breastfeeding should not be discouraged, but the levels are too low to treat or prevent TB in the infant. The infant should be assessed separately by a doctor, and breastfed infants should be observed for any effects. Do not stop prescribed TB treatment or breastfeeding without medical advice.

Isoniazid Brands, Strengths and Approximate Price in India

Examples of isoniazid brands marketed in India include Isonex and Isokin, based on pharmacy listings. This is not an exhaustive list, and availability can change. Listings show 100 mg and 300 mg tablets. These brands are not always available.

Approximate price in India (based on limited listings, so treat as an example only): ₹16–₹33 per strip of 10 tablets (300 mg)

Prices may vary by brand, strength, pack size, pharmacy and location, and the listings above come from a single online pharmacy. TB medicines may also be supplied free through India’s National TB Elimination Programme, so ask your doctor or TB programme provider. Isoniazid is a prescription medicine and should not be obtained or used without professional supervision.

How HealthPil Can Help

Medicines should be started, adjusted, or stopped only under the guidance of a qualified healthcare professional. If you have questions about isoniazid, TB treatment or your side effects, you can consult a doctor through HealthPil, including infectious disease specialists who can monitor your progress, review your symptoms and help manage your treatment safely.