Isoniazid: The Gold Standard in Tuberculosis Treatment
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Synonyms
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Isoniazid, also known by its chemical name isonicotinic acid hydrazide (INH), is a first-line antibacterial agent specifically indicated for the treatment and prevention of all forms of active tuberculosis (TB) infection caused by susceptible strains of Mycobacterium tuberculosis. As a cornerstone of antituberculosis therapy for over seven decades, it remains one of the most effective and widely used medications globally in the fight against this infectious disease. Its bactericidal activity, high efficacy during the intensive phase of treatment, and well-established safety profile when appropriately monitored make it an indispensable component of modern TB control programs. This medication is typically administered as part of a multi-drug regimen to prevent the development of drug resistance, adhering to global standards set by the World Health Organization and national health authorities.
Features
- Contains isoniazid as the active pharmaceutical ingredient
- Available in multiple formulations: oral tablets (100 mg, 300 mg), syrup (50 mg/5 mL), and injectable solutions
- Exhibits potent bactericidal activity against actively dividing mycobacteria
- Demonstrates good oral bioavailability with rapid absorption from the gastrointestinal tract
- Distributed widely throughout body tissues and fluids, including cerebrospinal fluid
- Primarily metabolized in the liver via acetylation, with genetic variations affecting metabolic rate
- Excreted mainly through the kidneys (75-95%) within 24 hours
- Manufactured under strict quality control standards complying with pharmacopeial requirements
Benefits
- Superior bactericidal action: Rapidly kills multiplying tubercle bacilli, reducing infectiousness quickly
- Prevention of resistance development: Essential component of combination therapy that prevents emergence of drug-resistant strains
- Proven efficacy in latent TB: Reduces the risk of active tuberculosis development by up to 90% when used as preventive therapy
- Excellent tissue penetration: Effectively reaches difficult-to-treat sites including meninges, caseous lesions, and cavities
- Convenient dosing regimen: Typically once-daily administration supports treatment adherence
- Cost-effective therapy: Decades of use and manufacturing have made it accessible in resource-limited settings
Common use
Isoniazid is primarily indicated for the treatment of all forms of active tuberculosis caused by susceptible strains of M. tuberculosis, always administered in combination with other antituberculosis medications such as rifampin, pyrazinamide, and ethambutol to prevent resistance development. It is equally crucial as monotherapy for the treatment of latent tuberculosis infection (LTBI) in individuals at high risk of progression to active disease, including recent contacts of infectious TB cases, HIV-positive individuals, patients with fibrotic changes on chest radiograph consistent with prior TB, and those receiving immunosuppressive therapy. The medication may also be used as part of post-exposure prophylaxis in specific high-risk scenarios following potential TB exposure.
Dosage and direction
Active tuberculosis treatment: 5 mg/kg (usual adult dose 300 mg) orally once daily, or 15 mg/kg (maximum 900 mg) orally 2-3 times weekly as part of directly observed therapy (DOT)
Latent tuberculosis treatment: 300 mg orally once daily for 6-9 months, or 900 mg orally twice weekly for 9 months under DOT
Pediatric dosing: 10-15 mg/kg (maximum 300 mg) orally once daily, or 20-30 mg/kg (maximum 900 mg) orally twice weekly
Administration should occur on an empty stomach, at least 1 hour before or 2 hours after meals, to maximize absorption. However, if gastrointestinal upset occurs, administration with a small amount of food may be acceptable despite slightly reduced bioavailability. Tablets may be crushed and mixed with food or liquid for patients with difficulty swallowing. For preventive therapy, treatment duration typically ranges from 6 to 9 months based on patient characteristics and regional guidelines.
Precautions
Routine monitoring of liver function tests (ALT, AST, bilirubin) is essential before initiation and periodically during treatment, as hepatotoxicity represents the most serious adverse effect. Patients should be educated to immediately report symptoms including fatigue, malaise, anorexia, nausea, vomiting, dark urine, or jaundice. Alcohol consumption should be avoided due to increased risk of hepatitis. Diabetic patients may require adjustment of hypoglycemic therapy as isoniazid may interfere with glucose metabolism. Peripheral neuropathy may develop, particularly in malnourished patients, alcoholics, diabetics, and those with HIV infection; pyridoxine (vitamin B6) supplementation is recommended for high-risk groups. Regular ophthalmological examinations are advised for patients receiving prolonged therapy due to potential optic neuritis.
Contraindications
Isoniazid is contraindicated in patients with a history of severe hypersensitivity reactions to the drug or any component of the formulation. It should not be administered to patients with acute liver disease of any etiology or a history of previous isoniazid-associated liver injury. The medication is contraindicated during acute episodes of gout due to potential elevation of uric acid levels. Concomitant administration with carbamazepine is contraindicated due to increased risk of both hepatotoxicity and reduced anticonvulsant efficacy. Use in patients with severe renal impairment requires careful consideration and dosage adjustment.
Possible side effect
Common effects (≥1%): Peripheral neuropathy (dose-related), elevated liver enzymes, nausea, vomiting, epigastric distress, dizziness, headache, fatigue
Less common effects (0.1-1%): Rash, fever, arthralgia, lupus-like syndrome, optic neuritis, tinnitus, urinary retention
Rare but serious effects (<0.1%): Clinical hepatitis (age-related risk), convulsions, psychoses, agranulocytosis, hemolytic anemia, aplastic anemia, hypersensitivity vasculitis, gynecomastia
Metabolic effects: Pyridoxine deficiency, pellagra-like syndrome, hyperglycemia, metabolic acidosis, hypocalcemia, hypophosphatemia
Drug interaction
Hepatotoxic agents: Enhanced risk of liver damage with concomitant use of acetaminophen, rifampin, pyrazinamide, anticonvulsants, or azole antifungals
Anticonvulsants: Decreases levels of carbamazepine and phenytoin; increases levels of ethosuximide and valproic acid
Benzodiazepines: May inhibit metabolism of diazepam and triazolam, increasing sedation
Theophylline: Isoniazid may increase theophylline levels, requiring monitoring
Disulfiram: Concurrent use may cause coordination difficulties and psychotic reactions
Corticosteroids: May decrease isoniazid levels through increased metabolism
Antacids: Aluminum-containing antacids may decrease absorption of isoniazid
Ketoconazole: Isoniazid may decrease ketoconazole levels significantly
Missed dose
If a dose is missed, it should be taken as soon as remembered unless it is almost time for the next scheduled dose. In that case, the missed dose should be skipped and the regular dosing schedule resumed. Patients should never double the dose to make up for a missed one. For those on twice-weekly regimens under directly observed therapy, healthcare providers should be notified immediately if a dose is missed to determine the appropriate course of action. Consistent adherence is critical for treatment success and prevention of resistance development.
Overdose
Isoniazid overdose constitutes a medical emergency that may rapidly progress to seizures, metabolic acidosis, coma, and death. Symptoms typically appear within 30 minutes to 3 hours and may include nausea, vomiting, slurred speech, dizziness, visual disturbances, hallucinations, hyperreflexia, and seizures refractory to standard anticonvulsants. Treatment involves immediate gastric lavage if presentation is early, activated charcoal administration, and aggressive supportive care. Pyridoxine (vitamin B6) is the specific antidote and should be administered intravenously in gram quantities equivalent to the amount of isoniazid ingested. Management should occur in an intensive care setting with continuous monitoring and respiratory support as needed.
Storage
Store at controlled room temperature (20-25°C or 68-77°F) in a tight, light-resistant container. Keep the medication in its original packaging to protect from moisture and light. Do not store in bathroom areas where humidity levels may fluctuate. Keep out of reach of children and pets. Do not use if the medication has expired or shows signs of physical deterioration. Oral solutions should not be frozen and should be discarded if discoloration or precipitation occurs. Proper disposal of unused medication should follow local regulations for pharmaceutical waste.
Disclaimer
This information is provided for educational purposes only and does not constitute medical advice. Isoniazid is a prescription medication that should only be used under the supervision of a qualified healthcare professional. Treatment decisions must be made by a physician based on individual patient characteristics, susceptibility testing, and local treatment guidelines. Patients should not initiate, adjust, or discontinue therapy without medical consultation. While every effort has been made to ensure accuracy, medical knowledge evolves and prescribing information may change. Healthcare providers should consult the most current official prescribing information before making treatment decisions.
Reviews
Clinical studies spanning decades have consistently demonstrated isoniazid’s efficacy in tuberculosis treatment, with success rates exceeding 95% when used appropriately in combination regimens. The medication receives high marks from infectious disease specialists for its reliable bactericidal activity and established safety profile with appropriate monitoring. Patients generally tolerate the medication well, though some report gastrointestinal discomfort during initial therapy. Public health authorities worldwide continue to endorse isoniazid as a fundamental component of TB control programs, citing its cost-effectiveness and proven impact on reducing disease transmission. Ongoing research continues to refine its role in the era of drug-resistant tuberculosis.