Prescription ·Strong evidence ·Reviewed May 2026
Isoniazid is a first-line antibiotic used to treat and prevent tuberculosis caused by Mycobacterium tuberculosis. It is highly bactericidal against actively dividing organisms and is given both for latent infection (as monotherapy) and active disease (in combination with other agents). It is one of the most important drugs in modern tuberculosis therapy.
The bottom line
Evidence rating strong. Most-documented uses: treatment of active pulmonary and extrapulmonary tuberculosis (in combination regimens), treatment of latent tuberculosis infection, prevention of tuberculosis in high-risk exposed individuals. 4 sources indexed (1980–2016), with 4 interaction records on file.
Core mechanism
Isoniazid is a prodrug activated by the mycobacterial enzyme KatG (catalase-peroxidase). The activated form inhibits InhA, an enoyl-acyl carrier protein reductase, blocking the synthesis of mycolic acids that are essential components of the mycobacterial cell wall. The resulting disruption of cell wall biosynthesis is bactericidal against replicating organisms. Structurally a pyridine analogue, isoniazid also competes with pyridoxine (vitamin B6) and interferes with its conversion to the active coenzyme pyridoxal 5'-phosphate, which underlies its characteristic depletion of B6 and the related impairment of niacin (vitamin B3) synthesis.3,1
Best absorbed on an empty stomach, at least 1 hour before or 2 hours after meals; food, and especially high-fat or high-carbohydrate meals, reduces and delays absorption. Antacids containing aluminum should be separated by at least 1 hour. May be taken with food if gastrointestinal upset occurs.
Nutrients this medication can lower over time, and what to replace.
Isoniazid antagonizes pyridoxine metabolism and increases risk of pyridoxine-responsive peripheral neuropathy.
Isoniazid is a hydrazide derivative that reacts directly with pyridoxal and pyridoxal-5'-phosphate to form isoniazid-pyridoxal hydrazones, which are inactive and rapidly excreted in the urine. Isoniazid also inhibits the enzyme pyridoxine phosphokinase, blocking conversion of dietary pyridoxine to its active coenzyme form. The combined effect functionally depletes active vitamin B6, impairing pyridoxal-5'-phosphate-dependent reactions including the synthesis of neurotransmitters and heme.
Niacin can be synthesized endogenously from the amino acid tryptophan via a pyridoxal-5'-phosphate (vitamin B6)-dependent pathway (the kynurenine pathway). By depleting active vitamin B6, isoniazid impairs this conversion of tryptophan to niacin. Isoniazid is also a structural analog of niacinamide and can interfere with niacin/NAD metabolism. The net effect can precipitate a niacin-deficiency (pellagra-like) state, particularly in patients with marginal dietary intake.
Isoniazid functionally antagonizes pyridoxine and can cause peripheral neuropathy; vitamin B6 supplementation prevents this in at-risk patients.
Recommendation: Use pyridoxine as prescribed, especially in pregnancy, diabetes, HIV, kidney disease, malnutrition, alcohol use, or neuropathy risk.
Isoniazid has weak monoamine oxidase inhibiting activity; 5-HTP could theoretically increase serotonergic adverse-effect risk, although the evidence is much weaker than for linezolid or antidepressant MAOIs.
Recommendation: Avoid nonessential 5-HTP during isoniazid therapy, especially with antidepressants or other serotonergic drugs; counsel on serotonin-toxicity symptoms if use is clinician-approved.
Isoniazid is a weak, reversible inhibitor of monoamine oxidase. Combining it with high doses of L-tryptophan, a precursor to serotonin, can increase serotonergic activity and theoretically contribute to serotonin excess, particularly when other serotonergic agents are also present.
Recommendation: Use caution when combining L-tryptophan with isoniazid, particularly at higher tryptophan doses or alongside other serotonergic medications. Watch for agitation, tremor, sweating, rapid heartbeat, or confusion and seek care if these occur. Discuss use with a clinician before starting.
Isoniazid carries a recognized risk of hepatotoxicity. Silymarin from milk thistle has been studied as a hepatoprotective adjunct in patients on antitubercular therapy, with some trials suggesting it may reduce markers of liver injury. Evidence is mixed and milk thistle is not a substitute for standard liver monitoring.
Recommendation: Milk thistle is not a replacement for routine liver function monitoring during isoniazid therapy. If considering it as a hepatoprotective adjunct, discuss with a clinician. Continue scheduled liver enzyme checks and report symptoms such as nausea, dark urine, jaundice, or right upper abdominal pain promptly.
Numbered references. Citations throughout the page link here.
Isoniazid remains a first-line agent for both latent and active drug-susceptible tuberculosis as part of standard multidrug regimens.
Isoniazid-associated hepatotoxicity increases with age, alcohol use, and concurrent hepatotoxins; baseline and periodic liver function monitoring is recommended.
Mutations in katG and inhA confer isoniazid resistance, confirming the prodrug-activation and mycolic-acid-synthesis-inhibition mechanism.
Supplemental pyridoxine (typically 25-50 mg daily) prevents peripheral neuropathy in patients at risk during isoniazid therapy.
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