Isoniazid

Pronunciation: eye-so-NYE-a-zid.

Generic name
isoniazid
Drug class
Antitubercular agent (antibiotic)
Form
Tablet (50 mg, 100 mg, or 300 mg, depending on market)
Route
Oral
Legal status
Prescription-only

Availability and supply rules vary by country

Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand

Australia: Isoniazid (generic); Noumed Isoniazid (5) · Full directory

What isoniazid is and how it works

Isoniazid is a prescription-only antibiotic tablet used to treat and prevent tuberculosis (TB). It is one of the oldest and most important first-line TB medicines, usually given together with other antitubercular drugs for active disease, or on its own for preventive (latent TB) treatment. (1) (2)

Isoniazid is taken by mouth, usually once daily, ideally on an empty stomach. Because it carries a real risk of serious liver problems, your prescriber will monitor you regularly throughout treatment, especially if you are 35 or older. It can also cause a form of nerve damage (peripheral neuropathy) related to vitamin B6 depletion, which is why many people are given a vitamin B6 (pyridoxine) supplement alongside isoniazid.

Mechanism of action: how isoniazid works

Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase enzyme KatG, which generates reactive species that inhibit synthesis of mycolic acids, essential components of the mycobacterial cell wall, leading to bactericidal activity against actively dividing Mycobacterium tuberculosis. Isoniazid is metabolised in the liver mainly by acetylation; genetically determined 'slow acetylator' status increases the risk of peripheral neuropathy and other toxicity. (1) (6) (4)

What isoniazid is used for

Active tuberculosis (treatment)

Used, together with other antitubercular medicines, to treat all forms of pulmonary (lung) and extrapulmonary (outside the lung) tuberculosis caused by susceptible organisms. (1) (2) (5)

Latent tuberculosis (preventive therapy)

Used on its own to prevent active tuberculosis from developing in people with latent (inactive) TB infection who are at higher risk, such as those with HIV, close contacts of someone with active TB, people who have recently tested positive on a TB skin test, or those with certain other risk factors. (1)

Before taking isoniazid

Do not take / not appropriate for

  • Do not take isoniazid if you have previously had isoniazid-associated liver injury, or a severe reaction to isoniazid such as drug fever, chills, or joint pain. (all countries) (1) (4)
  • Do not take isoniazid if you have acute liver disease of any cause. (United States) (1)

Tell your clinician about

  • Any history of liver disease, or previous liver problems with isoniazid or other medicines.
  • How much alcohol you drink -- daily alcohol use increases the risk of isoniazid-related liver injury.
  • Whether you have diabetes, are malnourished, have kidney problems (especially if on dialysis), are pregnant, or have HIV -- these increase the risk of peripheral neuropathy and may mean you need a vitamin B6 (pyridoxine) supplement.
  • All other medicines you take, especially phenytoin, carbamazepine, rifampin/rifampicin, or other medicines that can affect the liver.
  • Any history of seizures, mental health conditions, or vision problems.
  • Pregnancy, plans for pregnancy, or breastfeeding.
(1) (2) (6)

How to take isoniazid

  • Take isoniazid exactly as prescribed, usually once daily, ideally on an empty stomach (30 minutes before or 2 hours after food) unless your prescriber tells you otherwise.
  • Swallow tablets whole.
  • Complete the full course as prescribed -- stopping TB treatment early or missing doses can allow the infection to come back and can contribute to drug-resistant TB.
  • Do not change your dose or stop treatment on your own -- follow your prescriber's plan and attend all scheduled monitoring appointments.
(1) (2)

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6)

Missed dose

If you miss a dose, contact your prescriber or pharmacist for guidance rather than doubling your next dose -- consistent daily dosing matters for TB treatment to work properly and to reduce the risk of drug resistance.

Overdose

Isoniazid overdose is a medical emergency -- it can cause severe, hard-to-control seizures within 30 minutes to 3 hours of ingestion, along with severe metabolic disturbances. Seek emergency medical attention immediately; pyridoxine (vitamin B6) is a specific antidote used in hospital to help control isoniazid-overdose seizures. (6) (4)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Numbness, tingling, or a pins-and-needles feeling in the hands or feet (an early sign of peripheral neuropathy) -- more common in malnourished people, those with diabetes, alcohol use, kidney problems, or slow drug metabolism.

What to notice: Numbness, tingling, or a pins-and-needles feeling in the hands or feet (an early sign of peripheral neuropathy) -- more common in malnourished people, those with diabetes, alcohol use, kidney problems, or slow drug metabolism.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1) (4) (6)

Nausea, loss of appetite, or mild stomach upset.

What to notice: Nausea, loss of appetite, or mild stomach upset.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (6)

Mildly elevated liver enzymes on blood tests, which may resolve even if the drug is continued (your prescriber will monitor this).

What to notice: Mildly elevated liver enzymes on blood tests, which may resolve even if the drug is continued (your prescriber will monitor this).

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)

Serious warnings

Signs of liver problems (hepatitis) — boxed_warning

Symptoms: Fatigue, weakness, loss of appetite, nausea, vomiting, dark urine, or yellowing of the skin or eyes (jaundice).

Action: Contact your prescriber immediately and stop taking isoniazid if advised to -- severe and sometimes fatal liver damage has been reported with isoniazid, and the risk increases with age and daily alcohol use. You will be monitored regularly, including blood tests if you are 35 or older. (1) (4) (6)

Peripheral neuropathy (nerve damage) — high caution

Symptoms: Numbness, tingling, burning, or weakness, usually starting in the hands or feet.

Action: Tell your prescriber -- this is usually related to vitamin B6 (pyridoxine) depletion caused by isoniazid and is often prevented or treated with a pyridoxine supplement, especially if you are malnourished, have diabetes, drink alcohol regularly, have kidney problems, are pregnant, or have HIV. (1) (2) (4) (6)

Vision changes (optic neuritis) — high caution

Symptoms: Blurred vision, changes in colour vision, or eye pain.

Action: Contact your prescriber promptly for an eye assessment -- optic neuritis and, rarely, optic atrophy have been reported with isoniazid. (1) (6)

Seizures or significant mood/behaviour changes — high caution

Symptoms: New seizures, confusion, memory problems, or unusual thoughts or behaviour (toxic psychosis).

Action: Seek prompt medical attention -- convulsions, toxic encephalopathy, and toxic psychosis have been reported with isoniazid, particularly with higher doses or in susceptible individuals. (1) (6)

Interactions

  • Phenytoin: isoniazid can raise phenytoin blood levels, increasing the risk of phenytoin side effects/toxicity -- your prescriber may need to monitor levels and adjust your phenytoin dose. (1) (2) (6)
  • Carbamazepine: isoniazid can raise carbamazepine blood levels and increase carbamazepine toxicity, and can also increase isoniazid's own liver-toxic metabolites -- monitoring and dose adjustment may be needed. (2) (4) (6)
  • Rifampin/rifampicin: often used together for TB treatment, but the combination increases the risk of liver-related side effects -- your prescriber will monitor your liver function. (2) (4)
  • Alcohol: daily alcohol use increases the risk of isoniazid-related liver injury -- your prescriber may advise avoiding or reducing alcohol during treatment. (1) (2) (6)
  • Acetaminophen/paracetamol: isoniazid may increase the risk of liver toxicity from acetaminophen -- discuss safe use of pain/fever relievers with your prescriber. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Pregnancy

Isoniazid is used to treat active tuberculosis during pregnancy because the benefit of treating the mother's TB is considered to outweigh the potential risk to the unborn baby; animal studies have not shown isoniazid-related birth defects. Preventive (latent TB) treatment in pregnancy is weighed individually and is sometimes deferred until after delivery. Discuss with your prescriber. (1) (2) (3) (4) (5) (6)

Breastfeeding

Isoniazid passes into breast milk in small amounts that are not considered harmful to the infant, though the amount is not enough to protect the infant from TB on its own; breastfeeding is not generally discouraged. Discuss with your prescriber. (1) (2) (3) (4) (5) (6)

Children

Used in children from 3 months of age for active TB treatment and for TB prevention, with weight-based dosing determined by a specialist. (1) (2) (3) (4) (5) (6)

Elderly

People aged 50-64 have among the highest reported rates of isoniazid-related hepatitis, and monitoring (including liver blood tests) is especially important in this age group and older. (1) (2) (3) (4) (5) (6)

Hepatic_impairment

Isoniazid dosing may need to be reduced in people with liver impairment, and it should not be used in acute liver disease; liver function should be monitored closely. (1) (2) (3) (4) (5) (6)

Renal_impairment

Standard doses can generally be used in kidney impairment; people on dialysis should take their dose after the dialysis session, since isoniazid is removed by dialysis. (1) (2) (3) (4) (5) (6)

Source_ids

US-DAILYMED-ISONIAZID-2026,NZ-MEDSAFE-ISONIAZID-2026,CA-HC-ISONIAZID-2026 (1) (2) (3) (4) (5) (6)

Monitoring

Before starting

  • Baseline liver function tests, particularly in adults 35 and older or with other liver-risk factors
  • Assessment of alcohol use, nutritional status, diabetes, kidney disease, and pregnancy status (peripheral neuropathy risk factors)
  • Review of all other medicines, especially phenytoin, carbamazepine, and rifampin/rifampicin

During treatment

  • Monthly clinical review for symptoms of hepatitis (fatigue, weakness, loss of appetite, nausea, vomiting, dark urine, jaundice).
  • Periodic liver function blood tests, especially for patients 35 and older or with other risk factors.
  • Monitoring for peripheral neuropathy symptoms (numbness, tingling), with pyridoxine supplementation in higher-risk patients.
  • Monitoring for visual changes, seizures, or mood/behaviour changes.
(1) (2)

Storage

Store isoniazid tablets at room temperature, away from moisture and light, as directed on the product labelling. Keep out of reach of children.

Professional information

Clinician-facing context only, sourced from the current US FDA prescribing information and Health Canada/Medsafe product information; verify against the current full label in the relevant jurisdiction before prescribing.

United States

Indication: Active tuberculosis (with other antitubercular agents); latent TB preventive therapy

Dose summary: Treatment: adults 5 mg/kg up to 300 mg/day; children 10-15 mg/kg up to 300 mg/day. Prevention: adults >30 kg, 300 mg/day; children 10 mg/kg up to 300 mg/day. Hepatic enzyme monitoring recommended for patients 35 and older.

Renal context: Standard dosing generally used; dialysis patients should dose after dialysis (drug is removed by dialysis) per the New Zealand data sheet reviewed. (1)

United Kingdom

Indication: Pulmonary and extrapulmonary tuberculosis

Dose summary: Adults 4-5 mg/kg/day up to 300 mg/day (up to 10 mg/kg initially for TB meningitis); children from 3 months, 10-15 mg/kg/day. Withdraw treatment if AST exceeds 3x normal or bilirubin rises.

Renal context: Not specifically addressed with a dedicated adjustment in the source reviewed for this dossier. (2)

Mechanism of action

Hepatic guidance

Dose reduction may be needed in hepatic impairment; contraindicated in acute liver disease; close liver-function monitoring required, particularly in patients with pre-existing liver disease.

Overdose note

Isoniazid overdose can cause severe, refractory seizures and severe metabolic acidosis within 30 minutes to 3 hours of ingestion; intravenous pyridoxine (gram-for-gram equivalent to the amount ingested) is a specific antidote, together with short-acting barbiturates for seizure control and supportive care. (1) (6) (4)

Frequently asked questions

Is isoniazid available in the US, UK, Europe, Canada, Australia, and New Zealand?

Yes, isoniazid is a long-established, widely available antitubercular medicine in all of these regions. In the European Union it is authorised nationally by individual member states (for example, as NICOZID in Italy or Tibinide in Iceland/Sweden) rather than through a single centralised EU-wide approval. (1) (2) (3) (4) (5) (6)

Why does isoniazid carry a boxed (highest-level) warning about liver damage?

Isoniazid can cause severe, and in some cases fatal, liver damage (hepatitis). This risk increases with age -- it is higher in people over 35 -- and with daily alcohol use. It can develop even after many months of treatment, which is why regular monitoring, including liver blood tests for people 35 and older, continues throughout the course of treatment. (1) (4) (6)

Why am I also being given vitamin B6 (pyridoxine) with isoniazid?

Isoniazid can deplete the body's vitamin B6 (pyridoxine), which can lead to peripheral neuropathy -- numbness, tingling, or burning, usually starting in the hands or feet. People at higher risk (including those who are malnourished, have diabetes, drink alcohol regularly, have kidney problems, are pregnant, or have HIV) are often given a pyridoxine supplement alongside isoniazid to help prevent or treat this. (1) (2) (4) (6)

What is the mechanism of action of isoniazid?

Isoniazid is a prodrug activated by the mycobacterial catalase-peroxidase enzyme KatG, which generates reactive species that inhibit synthesis of mycolic acids, essential components of the mycobacterial cell wall, leading to bactericidal activity against actively dividing Mycobacterium tuberculosis. Isoniazid is metabolised in the liver mainly by acetylation; genetically determined 'slow acetylator' status increases the risk of peripheral neuropathy and other toxicity. (1) (6) (4)

What is the generic name of Isoniazid?

The generic name is isoniazid. (1)

What class of drug is isoniazid?

Isoniazid is classed as: Antitubercular agent (antibiotic). (1)

References

  1. ISONIAZID tablets, USP -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information, Teva Pharmaceuticals USA, Inc. · United States · accessed 2026-07-30 · source 1
  2. Isoniazid 100 mg Tablets -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom -- Morningside Healthcare Ltd · United Kingdom · accessed 2026-07-30 · source 2
  3. Isoniazid -- list of nationally authorised medicinal products, PSUSA/00001789/201911European Medicines Agency (EMA) PSUSA list of nationally authorised medicinal products · European Union · accessed 2026-07-30 · source 3
  4. Isoniazid Tablets USP -- Product MonographHealth Canada product monograph, PENDOPHARM, Division of Pharmascience Inc. · Canada · accessed 2026-07-30 · source 4
  5. ISONIAZID 100mg tablet -- ARTG registrationTherapeutic Goods Administration (TGA), Australia -- ARTG registration · Australia · accessed 2026-07-30 · source 5
  6. Isoniazid (PSM) -- New Zealand Data SheetMedsafe (New Zealand regulator), sponsor Noumed Pharmaceuticals Limited · New Zealand · accessed 2026-07-30 · source 6

Brand names by country

Single-ingredient isoniazid names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries

United States

  • Isoniazid (generic) (1)FDA approved currently labeled · 100 mg tablet, 300 mg tablet · Teva Pharmaceuticals USA, Inc.

United Kingdom

  • Isoniazid (generic) (2)MHRA licensed emc smpc confirmed · 100 mg tablet · Morningside Healthcare Ltd

Canada

  • ISOTAMINE-100
  • ISOTAMINE SYR

Australia

  • Isoniazid (generic); Noumed Isoniazid (5)TGA ARTG registered confirmed ARTG 13455 and 483125 · 100 mg tablet · Arrotex Pharmaceuticals Pty Ltd (also Noumed Pharmaceuticals Pty Ltd)

New Zealand

  • Isoniazid (PSM) (6)Medsafe approved confirmed via data sheet first approved 1969 · 100 mg tablet · Noumed Pharmaceuticals Limited
View brands in additional countries.

European Union

NICOZID (Italy); Tibinide (Iceland/Sweden); Isoniazid Tablets BP (3)

This page provides general reference information about isoniazid and is not a substitute for advice from the prescriber managing your treatment. All dosing, monitoring, and treatment decisions should be made with your prescriber.

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