An antibiotic pill for tuberculosis (TB). It is combined with other TB drugs to treat active disease and is also used to prevent TB in people at high risk. It can cause severe, sometimes fatal liver injury and nerve damage felt as tingling in the hands and feet.
Labeled use. Preventive therapy for high-risk tuberculin reactors and close contacts, regardless of age.
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Labeled use. All forms of susceptible TB; active disease always gets other antituberculosis drugs at the same time.
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Isoniazid is a backbone of first-line tuberculosis treatment, given with rifampin, pyrazinamide, and ethambutol in the initial phase. For latent TB infection, shorter rifamycin-based regimens (weekly isoniazid plus rifapentine for 3 months, or daily rifampin for 4 months) are now often preferred over 6 to 9 months of isoniazid alone because more people finish them. Rifampin alone avoids isoniazid hepatotoxicity and neuropathy but brings more drug interactions.
Inhibits synthesis of mycolic acids, an essential component of the mycobacterial cell wall, and kills actively growing M. tuberculosis both inside and outside cells. Peak levels occur 1 to 2 hours after an oral dose; it distributes into all body fluids including CSF, crosses the placenta, and 50% to 70% of a dose is excreted in urine within 24 hours. It is inactivated mainly by acetylation, which is genetically determined; slow acetylators reach higher levels and more toxicity. Resistance comes from mutations in katG, inhA, kasA, and ahpC and develops rapidly with monotherapy.
The label does not describe a narrow therapeutic index or direct dosing by measured blood levels of the drug.
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Doses come from the FDA label. Study tool, not for clinical decisions.
No dose table. Monitor carefully in severe renal dysfunction. Most reported cases of cerebellar syndrome occurred in chronic kidney disease.
Severe and sometimes fatal hepatitis can occur, even after many months of therapy. Risk is age related (about 23 per 1,000 at ages 50 to 64) and higher with daily alcohol use, chronic liver disease, injection drug use, and in women, particularly Black and Hispanic women and after delivery. Interview patients monthly; for those 35 and older, measure AST and ALT before starting and periodically. Strongly consider stopping if liver tests rise beyond 3 to 5 times the normal upper limit, stop promptly for symptoms of liver injury, and defer preventive therapy in acute liver disease.
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Pyridoxine (vitamin B6) given IV. If the amount ingested is known, give pyridoxine gram for gram equal to the isoniazid dose as a slow IV bolus over 3 to 5 minutes; if unknown, start with 5 g in adults or 80 mg/kg in children, and repeat if seizures continue. Add diazepam for seizures that do not respond, support ventilation, and correct metabolic acidosis. Symptoms start 30 minutes to 3 hours after ingestion, and 80 to 150 mg/kg can be fatal if untreated.
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Embryocidal in rats and rabbits but not teratogenic in animal studies; there are no adequate human studies. Treat active tuberculosis in pregnancy, because the benefit justifies the risk; preventive therapy is generally started after delivery. It crosses the placenta, so watch exposed newborns for adverse effects. Hepatitis risk may be higher after delivery.
The small amounts in breast milk do not harm the nursing newborn, so breastfeeding should not be discouraged. Milk levels are too low to treat or protect the infant.
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Treatment: 10 to 15 mg/kg up to 300 mg once daily, or 20 to 40 mg/kg (maximum 900 mg/day) two or three times weekly. Preventive therapy: 10 mg/kg up to 300 mg once daily, or 20 to 30 mg/kg (no more than 900 mg) twice weekly under direct observation when daily adherence cannot be assured. Hepatitis is least common under age 20. Miliary TB, bone or joint TB, and TB meningitis in infants and children get 12 months of therapy.
No separate geriatric section. Hepatitis risk is age related, highest at about 23 per 1,000 for ages 50 to 64, and progressive liver damage occurs in up to 2.3% of people over 50. The label calls for careful monitoring over age 35, with AST and ALT before and during therapy.