Class: Antituberculosis agent
- Injection 100 mg/mL
- Tablets 100 mg
- Tablets 300 mg
- Tablets 500 mg
- Syrup 50 mg/5 mL
Interferes with lipid and nucleic acid biosynthesis in actively growing tubercle bacilli.
T max is 1 to 2 h.
Diffuses readily into cerebrospinal, pleural, and ascitic fluids, tissues, organs, saliva, sputum, feces, placental barrier, and in breast milk.
Primarily by acetylation and dehydrazination.
50% to 70% excreted in the urine in 24 h.
Indications and Usage
Treatment of all forms of tuberculosis.
Improvement of severe tremor in multiple sclerosis.
Previous isoniazid-associated hepatic injury, drug fever, chills, or arthritis; acute liver disease.
Dosage and AdministrationTuberculosis
PO / IM 5 mg/kg/day as single daily dose (max, 300 mg/day) or 15 mg/kg 2 to 3 times/wk (max, 900 mg).Infants and Children
PO / IM 10 to 20 mg/kg/day in single daily dose (max, 300 mg/day) or 20 to 40 mg/kg 2 or 3 times/week (max, 400 mg).
Drug InteractionsAluminum salts
May reduce oral absorption of isoniazid; give isoniazid 1 to 3 h before aluminum salts.Carbamazepine
May result in carbamazepine toxicity or isoniazid hepatotoxicity. Monitor carbamazepine concentrations and liver function.Disulfiram
May result in increased incidence of CNS effects (eg, coordination difficulties, confusion, irritability, aggressiveness).Enflurane
May result in high-output renal failure in rapid acetylators. Monitor renal function.Hydantoins
May increase serum hydantoin levels.Rifampin
May result in higher rate of hepatotoxicity.
Laboratory Test Interactions
None well documented.
Peripheral neuropathy; convulsions; toxic encephalopathy; optic neuritis and atrophy; memory impairment; toxic psychosis.
Morbilliform, maculopapular, purpuric, or exfoliative skin eruptions.
Nausea; vomiting; epigastric distress.
Agranulocytosis; hemolytic, sideroblastic, or aplastic anemia; thrombocytopenia; eosinophilia.
Hepatotoxicity, including elevated serum transaminase levels, bilirubinemia, bilirubinuria, jaundice, severe and sometimes fatal hepatitis.
Pyridoxine deficiency; pellagra; hyperglycemia; metabolic acidosis; hypocalcemia; hypophosphatemia.
Gynecomastia; rheumatic syndrome; systemic lupus erythematosus-like syndrome; local irritation at IM injection site.
Severe and sometimes fatal hepatitis may occur during and many mo after treatment. Risk is related to age and increased with daily alcohol consumption. If reinitiated, start in small and gradual doses. Not for use in patients with active liver disease. Careful monitoring and monthly interviews are recommended.
Excreted in breast milk.
Discontinue drug at first sign of hypersensitivity reaction. Restart only after symptoms have cleared.
Monitor patients with severe renal function impairment carefully.
Common prodromal symptoms of hepatotoxicity include anorexia, nausea, vomiting, fatigue, malaise, and weakness. Patients with acute hepatic disease should have preventive tuberculosis treatment deferred. Incidence of hepatic reaction increases in patients older than 50 yr of age.
Prophylactic coadministration of pyridoxine (6 to 50 mg/day) is recommended in malnourished patients and those predisposed to neuropathy (eg, alcoholics, diabetics).
Nausea, vomiting, dizziness, slurring of speech, blurring of vision, visual hallucinations, respiratory distress, CNS depression, stupor, coma, severe seizures.
- Advise patient to minimize daily alcohol consumption while taking isoniazid because of the increased risk of hepatitis.
- Instruct patient to report the following symptoms to health care provider: weakness; fatigue; loss of appetite; nausea and vomiting; yellowing of skin or eyes; darkening of urine; numbness or tingling in hands or feet.
- Emphasize to patient that treatment will be lengthy and that patient must complete entire course of therapy. Relapse of tuberculosis is higher if chemotherapy is discontinued prematurely.
- Advise patient to return for laboratory follow-up.
- Caution patient not to perform activities that require mental alertness if adverse CNS symptoms occur.
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