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Quick facts
- Drug class
- Antimycobacterial [EPC]
- How it is given
- oral
- Type
- human prescription drug
- RxNorm ID
- 311166
We do not publish dosing. How much of this medicine to take, how often, and for how long depends on your age, weight, kidney and liver function, other conditions and other medicines. That is a decision for the person who prescribes or dispenses it. Follow the directions on your own pack and ask your pharmacist if anything is unclear.
What it is used to treat
INDICATIONS AND USAGE Isoniazid tablets, USP are recommended for all forms of tuberculosis in which organisms are susceptible. However, active tuberculosis must be treated with multiple concomitant anti-tuberculosis medications to prevent the emergence of drug resistance. Single-drug treatment of active tuberculosis with isoniazid or any other medication, is inadequate therapy. Isoniazid tablets, USP are recommended as preventive therapy for the following groups, regardless of age. (Note: the criterion for a positive reaction to a skin test (in millimeters of induration) for each group is given in parenthesis): Persons with human immunodeficiency virus (HIV) infection (greater than or equal to 5 mm) and persons with risk factors for HIV infection whose HIV infection status is unknown but who are suspected of having HIV infection. Preventive therapy may be considered for HIV infected persons who are tuberculin-negative but belong to groups in which the prevalence of tuberculosis infection is high. Candidates for preventive therapy who have HIV infection should have a minimum of 12 months of therapy. Close contacts of persons with newly diagnosed infectious tuberculosis (greater than or equal to 5 mm). In addition, tuberculin-negative (less than 5 mm) children and adolescents who have been close contacts of infectious persons within the past 3 months are candidates for preventive therapy until a repeat tuberculin skin test is done 12 weeks after contact with the infectious source. If the repeat skin test is positive (greater than 5mm), therapy should be continued. Recent converters, as indicated by a tuberculin skin test (greater than or equal to 10 mm increase within a 2-year period for those less than 35 years old; greater than or equal to 15 mm increase for those greater than or equal to 35 years of age). All infants and children younger than 4 years of age with a greater than 10 mm skin test are included in this category. Persons with abnormal chest radiographs that show fibrotic lesions likely to represent old healed tuberculosis (greater than or equal to 5 mm). […]
About this medicine
DESCRIPTION Isoniazid, USP is an antibacterial available as 100 mg and 300 mg tablets for oral administration. Each tablet also contains as inactive ingredients: colloidal silicon dioxide, crospovidone, hydrogenated vegetable oil, microcrystalline cellulose, pregelatinized corn starch and talc. Isoniazid, USP is chemically known as isonicotinyl hydrazine or isonicotinic acid hydrazide. It has the following structural formula: C 6 H 7 N 3 O M.W. 137.14 Isoniazid, USP is odorless, and occurs as a colorless or white crystalline powder or as white crystals. It is freely soluble in water, sparingly soluble in alcohol and slightly soluble in chloroform and in ether. Isoniazid, USP is slowly affected by exposure to air and light.
How it works
Mechanism of Action Isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bactericidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms.
Who should not take it
CONTRAINDICATIONS Isoniazid is contraindicated in patients who develop severe hypersensitivity reactions, including drug-induced hepatitis; previous isoniazid-associated hepatic injury; severe adverse reactions to isoniazid such as drug fever, chills, arthritis; and acute liver disease of any etiology.
Warnings
WARNINGS See the boxed warning . Severe Cutaneous Adverse Reactions Severe cutaneous adverse reactions (SCARs) including toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome (SJS), drug reaction with eosinophilia and systemic symptoms (DRESS), and acute generalized exanthematous pustulosis (AGEP) have been reported with the use of isoniazid (see ADVERSE REACTIONS ). Symptoms can be serious and potentially life threatening. If symptoms or signs of SCARs develop, discontinue isoniazid tablets immediately and institute appropriate therapy. Cerebellar Syndrome Cerebellar syndrome which may include abnormal motor coordination presenting as gait, trunk, and limb ataxia, dysmetria and dysdiadochokinesia, intention tremor, dysarthria, or nystagmus, has been reported in postmarketing case reports with the use of isoniazid (see ADVERSE REACTIONS ). Most cases of cerebellar syndrome involved patients with chronic kidney disease (CKD), however, cerebellar syndrome was also reported in patients without CKD. Discontinue isoniazid tablets if symptoms or signs of cerebellar syndrome occur.
Reported side effects
ADVERSE REACTIONS The most frequent reactions are those affecting the nervous system and the liver. Nervous System Reactions Peripheral neuropathy is the most common toxic effect. It is dose-related, occurs most often in the malnourished and in those predisposed to neuritis (e.g., alcoholics and diabetics) and is usually preceded by paresthesias of the feet and hands. The incidence is higher in "slow inactivators". Other neurotoxic effects, which are uncommon with conventional doses, are convulsions, toxic encephalopathy, optic neuritis and atrophy, memory impairment and toxic psychosis. Cerebellar syndrome, which may include abnormal motor coordination manifesting as gait, trunk, and limb ataxia, dysmetria and dysdiadochokinesia, intention tremor, dysarthria, or nystagmus, have been reported in post marketing case reports (see WARNINGS ). Hepatic Reactions See boxed warning . Elevated serum transaminase (SGOT; SGPT), bilirubinemia, bilirubinuria, jaundice and occasionally severe and sometimes fatal hepatitis. The common prodromal symptoms of hepatitis are anorexia, nausea, vomiting, fatigue, malaise and weakness. Mild hepatic dysfunction, evidenced by mild and transient elevation of serum transaminase levels occurs in 10 to 20 percent of patients taking isoniazid. This abnormality usually appears in the first 1 to 3 months of treatment but can occur at any time during therapy. In most instances, enzyme levels return to normal and generally, there is no necessity to discontinue medication during the period of mild serum transaminase elevation. In occasional instances, progressive liver damage occurs, with accompanying symptoms. If the SGOT value exceeds three to five times the upper limit of normal, discontinuation of the isoniazid should be strongly considered. The frequency of progressive liver damage increases with age. It is rare in persons under 20, but occurs in up to 2.3 percent of those over 50 years of age. Gastrointestinal Reactions Nausea, vomiting, epigastric distress, and pancreatitis. Hematologic Reactions Agranulocytosis; hemolytic, sideroblastic or aplastic anemia, thrombocytopenia; and eosinophilia. […]
Interactions with other medicines
Drug Interactions Food Isoniazid should not be administered with food. Studies have shown that the bioavailability of isoniazid is reduced significantly when administered with food. Tyramine- and histamine-containing foods should be avoided in patients receiving isoniazid. Because isoniazid has some monoamine oxidase inhibiting activity, an interaction with tyramine-containing foods (cheese, red wine) may occur. Diamine oxidase may also be inhibited, causing exaggerated response (e.g., headache, sweating, palpitations, flushing, hypotension) to foods containing histamine (e.g., skipjack, tuna, other tropical fish). Acetaminophen A report of severe acetaminophen toxicity was reported in a patient receiving isoniazid. It is believed that the toxicity may have resulted from a previously unrecognized interaction between isoniazid and acetaminophen and a molecular basis for this interaction has been proposed. However, current evidence suggests that isoniazid does induce P-450IIE1, a mixed-function oxidase enzyme that appears to generate the toxic metabolites, in the liver. Furthermore it has been proposed that isoniazid resulted in induction of P-450IIE1 in the patient's liver which, in turn, resulted in a greater proportion of the ingested acetaminophen being converted to the toxic metabolites. Studies have demonstrated that pretreatment with isoniazid potentiates acetaminophen hepatotoxicity in rats 1,2 . Carbamazepine Isoniazid is known to slow the metabolism of carbamazepine and increase its serum levels. Carbamazepine levels should be determined prior to concurrent administration with isoniazid, signs and symptoms of carbamazepine toxicity should be monitored closely and appropriate dosage adjustment of the anticonvulsant should be made 3 . Ketoconazole Potential interaction of ketoconazole and isoniazid may exist. When ketoconazole is given in combination with isoniazid and rifampin the AUC of ketoconazole is decreased by as much as 88 percent after 5 months of concurrent isoniazid and rifampin therapy 4 . Phenytoin Isoniazid may increase serum levels of phenytoin. […]
If too much is taken
OVERDOSAGE Signs and Symptoms Isoniazid overdosage produces signs and symptoms within 30 minutes to 3 hours after ingestion. Nausea, vomiting, dizziness, slurring of speech, blurring of vision and visual hallucinations (including bright colors and strange designs) are among the early manifestations. With marked overdosage, respiratory distress and CNS depression, progressing rapidly from stupor to profound coma, are to be expected, along with severe, intractable seizures. Severe metabolic acidosis, acetonuria and hyperglycemia are typical laboratory findings. Treatment Untreated or inadequately treated cases of gross isoniazid overdosage, 80 mg/kg to 150 mg/kg, can cause neurotoxicity6 and terminate fatally, but good response has been reported in most patients brought under adequate treatment within the first few hours after drug ingestion. For the Asymptomatic Patient Absorption of drugs from the GI tract may be decreased by giving activated charcoal. Gastric emptying should also be employed in the asymptomatic patient. Safeguard the patient's airway when employing these procedures. Patients who acutely ingest greater than 80 mg/kg should be treated with intravenous pyridoxine on a gram per gram basis equal to the isoniazid dose. If an unknown amount of isoniazid is ingested, consider an initial dose of 5 grams of pyridoxine given over 30 to 60 minutes in adults or 80 mg/kg of pyridoxine in children. For the Symptomatic Patient Ensure adequate ventilation, support cardiac output and protect the airway while treating seizures and attempting to limit absorption. If the dose of isoniazid is known, the patient should be treated initially with a slow intravenous bolus of pyridoxine, over 3 to 5 minutes, on a gram per gram basis, equal to the isoniazid dose. If the quantity of isoniazid ingestion is unknown, then consider an initial intravenous bolus of pyridoxine of 5 grams in the adult or 80 mg/kg in the child. If seizures continue, the dosage of pyridoxine may be repeated. It would be rare that more than 10 grams of pyridoxine would need to be given. The maximum safe dose for pyridoxine in isoniazid intoxication is not known. […]
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This page is built from the openFDA record of the U.S. Food and Drug Administration's approved labelling for this product, effective 16 February 2026. Label text has been shortened and reformatted for readability; safety wording has not been altered in meaning. FDA labelling describes products approved in the United States — brand names, strengths, formulations and approved uses may differ in your country. Always read the leaflet that comes with your own pack. PocketsInfo is not affiliated with, and is not endorsed by, the FDA. Retrieved 2026-07-30.