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Isoniazid chemical structure

IUPAC name
CAS number
ATC code


Chemical formula {{{chemical_formula}}}
Molecular weight 137.139 g/mol
Metabolism liver; CYP450: 2C19, 3A4 inhibitor
Elimination half-life 0.5-1.6h (fast acetylators), 2-5h (slow acetylators)
Excretion urine (primarily), feces
Pregnancy category C
Legal status prescription only (US)
Routes of administration oral, intramuscular, intravenous

Isoniazid (also called isonicotinyl hydrazine or INH) is an organic compound that is the first-line antituberculosis medication in prevention and treatment. Isoniazid is never used on its own to treat active tuberculosis because resistance quickly develops.

Isoniazid is used in the treatment of mycobacterial infection. It was reported in 1952 by Roche (trademarked as Rimifon), when for the first time, a cure for tuberculosis was considered reasonable. It is available in tablet, syrup, and injectable forms (given intramuscularly or intravenously). Isoniazid is available worldwide, is inexpensive and is generally well tolerated. It is manufactured from isonicotinic acid, which is produced from 4-methylpyridine.[1]

Mechanism of action

Isoniazid is a prodrug and must be activated by bacterial catalase.[2] It is activated by catalase-peroxidase enzyme KatG which couples the isonicotinic acyl with NADH to form isonicotinic acyl-NADH complex. This complex binds tightly to ketoenoylreductase known as InhA, thereby blocking the natural enoyl-AcpM substrate. This process inhibits the synthesis of mycolic acid required for the mycobacterial cell wall.

Isoniazid reaches therapeutic concentrations in serum, cerebrospinal fluid (CSF), and within caseous granulomas. Isoniazid is metabolized in the liver via acetylation. There are two forms of the enzyme responsible for acetylation, so that some patients metabolize the drug more quickly than others. Hence, the half-life is bimodal with peaks at 1 hour and 3 hours in the US population. The metabolites are excreted in the urine. Doses do not usually have to be adjusted in case of renal failure.

Isoniazid is bactericidal to rapidly-dividing mycobacteria but is bacteriostatic if the mycobacterium is slow-growing.


The standard dose of isoniazid in adults is 5mg/kg/day (max 300mg daily). When prescribed intermittently (twice or thrice weekly) the dose is 15mg/kg (max 900mg daily). Patients with slow clearance of the drug (via acetylation as described above) may require reduced dosages to avoid toxicity.

Side effects

Adverse reactions include rash, abnormal liver function tests, hepatitis, sideroblastic anemia, peripheral neuropathy, mild central nervous system (CNS) effects, drug interactions resulting in increased phenytoin (Dilantin) or disulfiram (Antabuse) levels and intractable seizures (status epilepticus).

Peripheral neuropathy and CNS effects are associated with the use of isoniazid and are due to pyridoxine (vitamin B6) depletion, but are uncommon at doses of 5 mg/kg. Persons with conditions in which neuropathy is common (e.g., diabetes, uremia, alcoholism, malnutrition, HIV-infection), as well as pregnant women and persons with a seizure disorder, may be given pyridoxine (vitamin B6) (10-50 mg/day) with isoniazid.

Hepatotoxicity can be avoided with close clinical monitoring of the patient, specifically nausea, vomiting, abdominal pain and appetite. Isoniazid is metabolized by the liver mainly by acetylation and dehydrazination. The N-acetylhydrazine metabolite is believed to be responsible for the hepatotoxic effects seen in patients treated with isoniazid. The rate of acetylation is genetically determined. Approximately 50% of blacks and Caucasians are slow inactivators; the majority of Inuit and Asians are rapid inactivators. The half-life in fast acetylators is 1 to 2 hours while in slow acetylators it is 2 to 5 hours. Elimination is largely independent of renal function, however the half-life may be prolonged in liver disease. The rate of acetylation has not been shown to significantly alter the effectiveness of isoniazid. However, slow acetylation may lead to higher blood concentrations with chronic administration of the drug, with an increased risk of toxicity. Isoniazid and its metabolites are excreted in the urine with 75 to 95% of the dose excreted in 24 hours. Small amounts are also excreted in saliva, sputum and feces. Isoniazid is removed by hemodialysis and peritoneal dialysis.[1] Headache, poor concentration, poor memory and depression have all been associated with isoniazid use. The frequency of these side effects is not known, and the association with isoniazid is not well validated. The presence of these symptoms is not frequently disabling and is certainly not a reason to stop treatment with isoniazid; the patient should be strongly encouraged to continue treatment despite these symptoms. It must be explained to the patient that the harm done from not taking isoniazid far outweighs the problems arising from these symptoms.

INH therapy will decrease the efficacy of hormonal birth control when combined with Rifampin.

Synonyms and abbreviations

  • Isonicotinyl hydrazine
  • Isonicotinic acid hydrazide
  • INH
  • H (for "hydrazide", and also the WHO standard abbreviation)

See also

  • Tuberculosis treatment
  • Tuberculosis
  • Mycobacterium tuberculosis


  1. Shinkichi Shimizu, Nanao Watanabe, Toshiaki Kataoka, Takayuki Shoji, Nobuyuki Abe, Sinji Morishita, Hisao Ichimura "Pyridine and Pyridine Derivatives" in "Ullmann's Encyclopedia of Industrial Chemistry" 2007; John Wiley & Sons: New York.
  2. Timmins GS, Deretic V (2006). Mechanisms of action of isoniazid. Mol. Microbiol. 62 (5): 1220–7.
See Chapter 6, Treatment of LTBI Regimens - Isoniazid::
See Chapter 7 - Treatment of TB Disease Monitoring - Adverse Reactions to First-Line TB Drugs - Isoniazid::
See Table 5 First-Line Anti-TB Medications
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