Isoniazid is an antibiotic used to treat infection with mycobacteria (tuberculosis bacteria). In combination with other antimycobacterial drugs, it is most commonly used to treat tuberculosis.

Although the active ingredient was synthesized as early as 1912, its antibiotic effect was not recognized until after World War II, and the drug was introduced to the market in the early 1950s.

Since a deficiency of vitamin B6 can occur during therapy, the antibiotic should be administered together with a vitamin B6 preparation in order to counteract neurological side effects.



Isoniazid has a bactericidal (killing) effect on rapidly multiplying mycobacteria and a bacteriostatic (growth-inhibiting) effect on slowly multiplying ones. The antibiotic is administered as an inactive precursor and must first be activated by the bacterial enzyme KatG. Instead of nicotinic acid, the active isonicotinic acid is now incorporated into the cofactor NAD. This inhibits the synthesis of mycolic acids, which are an essential component of the bacterial cell wall.

During treatment, the active ingredient interferes with the body's own pyridoxine (vitamin B6) metabolism, affecting the build-up or formation of neurotransmitters such as GABA, serotonin or catecholamines.

Isoniazid should be used only in combination with other drugs, since the antifungal agent develops 46-89% resistance after two to three months of therapy alone and virtually 100% resistance of the pathogens after six to eight months.


Isoniazid is absorbed primarily in the small intestine after oral administration. The absorption and bioavailability of the active ingredient are reduced when taken concomitantly with food; therefore, the drug should be taken in a single dose on an empty stomach.
Der Stoffwechsel findet primär in der Leber statt, wo Isoniazid durch Acetylierung und Hydrolyse zu inaktiven Zwischenprodukten abgebaut wird. Die genetisch festgelegte Geschwindigkeit, mit der das Antibiotikum acetyliert wird, bestimmt die Plasmakonzentration sowie die Halbwertszeit. Letztere beträgt je nach Acetylierungsrate 0,5-1,6 Stunden bzw. 2-5 Stunden.
Der Großteil einer Isoniazid-Dosis wird innerhalb eines Tages mit dem Urin ausgeschieden.


Isoniazid should not be used in hypersensitivity, peripheral nervous system disorders, severe bleeding tendencies, and liver dysfunction (hepatitis, cirrhosis).

Drug interactions

Interactions may occur with drugs such as barbiturates, phenytoin, carbamazepine, primidone, rifampicin, valproic acid, acetaminophen, ketoconazole, theophylline, disulfiram, and levodopa.

During use, foods containing histamine and thyramine and alcohol should be avoided, and caffeine intake should be limited.
Isoniazid sollte getrennt von Antazida (Magensaftneutralisierende Medikamente) eingenommen werden.


Side effects

Side effects include:

  • skin rashes, itching
  • Muscle aging
  • elevated liver enzymes
  • hepatitis (inflammation of the liver)
  • Gastrointestinal disorders (including diarrhea, constipation, vomiting)
  • Peripheral nervous system disorders (when taken without vitamin B6)

Toxicological data

LD50 (human, oral): 100 mg/kg

Chemical & physical properties

ATC Code J04AC01
Formula C6H7N3O
Molar Mass (g·mol−1) 137,1393
Physical State solid
Melting Point (°C) 171,4
PKS Value 1,82
CAS Number 54-85-3
PUB Number DB00951
Drugbank ID DB00951

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All information used for the content comes from verified sources (recognised institutions, experts, studies by renowned universities). We attach great importance to the qualification of the authors and the scientific background of the information. Thus, we ensure that our research is based on scientific findings.
Markus Falkenstätter, BSc

Markus Falkenstätter, BSc

Markus Falkenstätter is a writer on pharmaceutical topics in Medikamio's medical editorial team. He is in the last semester of his pharmacy studies at the University of Vienna and loves scientific work in the field of natural sciences.

Mag. pharm. Stefanie Lehenauer

Mag. pharm. Stefanie Lehenauer

Stefanie Lehenauer has been a freelance writer for Medikamio since 2020 and studied pharmacy at the University of Vienna. She works as a pharmacist in Vienna and her passion is herbal medicines and their effects.

The content of this page is an automated and high-quality translation from DeepL. You can find the original content in German here.


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