Isoniazid is a first-line antituberculosis medication that represents the cornerstone of modern TB therapy. As a bactericidal antibiotic specifically targeting Mycobacterium tuberculosis, it demonstrates exceptional efficacy in both active tuberculosis treatment and latent infection prevention. Its unique mechanism of action, excellent tissue penetration, and well-established safety profile have maintained its position as an essential component of global TB control programs for over six decades. This medication continues to save millions of lives worldwide through systematic implementation in both individual patient care and public health initiatives.

Features

• Bactericidal activity against actively dividing mycobacteria
• High oral bioavailability with rapid absorption
• Excellent penetration into body tissues and cavities
• Specificity for Mycobacterium tuberculosis complex
• Available in multiple formulations: tablets, syrup, and injectable
• Cost-effective manufacturing and widespread availability
• Compatibility with most antiretroviral therapies
• Well-characterized pharmacokinetic profile

Benefits

• Achieves rapid reduction in bacterial load during initial treatment phase
• Prevents development of active tuberculosis in exposed individuals
• Reduces transmission rates within communities through effective treatment
• Demonstrates high efficacy when used as part of combination therapy
• Enables shorter treatment durations compared to historical regimens
• Supports global tuberculosis elimination efforts through prophylactic use

Common use

Isoniazid is primarily indicated for the treatment of all forms of active tuberculosis caused by susceptible strains of Mycobacterium tuberculosis. It must always be administered in combination with other antituberculosis drugs to prevent development of resistance. Additionally, it is widely used as preventive therapy for individuals with latent tuberculosis infection who are at high risk of developing active disease. This includes recent contacts of infectious TB cases, HIV-positive individuals, patients undergoing immunosuppressive therapy, and those with radiographic evidence of previous tuberculosis. The medication also finds application in certain cases of nontuberculous mycobacterial infections when susceptibility is confirmed.

Dosage and direction

Active Tuberculosis Treatment: Adults: 5 mg/kg (maximum 300 mg) daily or 15 mg/kg (maximum 900 mg) twice weekly Children: 10-15 mg/kg (maximum 300 mg) daily or 20-40 mg/kg (maximum 900 mg) twice weekly

Latent Tuberculosis Treatment: Adults: 300 mg daily for 6-9 months or 900 mg twice weekly for 6-9 months Children: 10-20 mg/kg (maximum 300 mg) daily for 9 months

Administration should occur on an empty stomach, preferably 1 hour before or 2 hours after meals, to optimize absorption. Dosage adjustments are necessary in patients with impaired renal function, while hepatic impairment requires careful monitoring rather than automatic dose reduction. Twice-weekly dosing must be administered under directly observed therapy (DOT) to ensure compliance.

Precautions

Baseline assessment should include complete blood count, liver function tests, and renal function evaluation. Regular monitoring of hepatic enzymes is mandatory throughout treatment, with frequency increased to monthly testing in patients with pre-existing liver disease or those consuming alcohol regularly. Patients should be educated to report immediately any symptoms of hepatitis, peripheral neuropathy, or visual disturbances. Pyridoxine supplementation (25-50 mg daily) is recommended for all patients at risk of neuropathy, including those with diabetes, HIV infection, malnutrition, pregnancy, breastfeeding women, and patients with chronic alcohol use. Special caution is required in elderly patients due to increased risk of hepatitis.

Contraindications

Absolute contraindications include previous severe hypersensitivity reactions to isoniazid, acute liver disease of any etiology, or severe hepatic damage from previous isoniazid administration. Relative contraindications include chronic liver disease, alcoholism, history of isoniazid-associated hepatitis, peripheral neuropathy, and convulsive disorders. The medication should not be administered to patients taking certain anticonvulsants without careful monitoring and dosage adjustment. Concomitant use with other hepatotoxic drugs requires extreme caution and frequent monitoring.

Possible side effect

Common (≥1%): Elevated liver enzymes, peripheral neuropathy, nausea, vomiting, epigastric discomfort, dizziness, rash, fever Less common (0.1-1%): Hepatitis, optic neuritis, psychoses, convulsions, hematologic abnormalities Rare (<0.1%): Lupus-like syndrome, arthralgia, gynecomastia, metabolic acidosis, pellagra-like syndrome Hepatotoxicity typically presents with fatigue, malaise, anorexia, nausea, and dark urine, progressing to jaundice in severe cases. Peripheral neuropathy manifests as symmetrical numbness and tingling in extremities. Regular monitoring can detect most adverse effects before they become severe.

Drug interaction

Isoniazid inhibits hepatic microsomal enzymes, particularly CYP2C19 and CYP3A4, leading to numerous significant interactions. It increases concentrations of phenytoin, carbamazepine, benzodiazepines, and theophylline, requiring dosage reduction and careful monitoring. Concurrent administration with rifampin increases hepatotoxicity risk. Alcohol consumption potentiates both hepatotoxicity and peripheral neuropathy. Antacids containing aluminum hydroxide decrease isoniazid absorption. Interaction with disulfiram may cause coordination difficulties and psychotic episodes. Dose adjustment of warfarin is frequently necessary due to enhanced anticoagulant effect.

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 patients on twice-weekly therapy under directly observed treatment, missed doses require special management according to public health guidelines, often involving additional dosing or extended treatment duration to ensure adequate therapy completion.

Overdose

Acute isoniazid overdose represents a medical emergency requiring immediate hospitalization. Symptoms typically appear within 30 minutes to 3 hours and include nausea, vomiting, slurred speech, dizziness, visual disturbances, and seizures that may progress to status epilepticus. Metabolic acidosis is common and may be severe. Treatment involves gastric lavage if presentation is early, activated charcoal administration, and aggressive supportive care. Pyridoxine (vitamin B6) is the specific antidote, administered gram-for-gram with the amount of isoniazid ingested. Intravenous benzodiazepines are used for seizure control. Hemodialysis may be beneficial in severe cases.

Storage

Store at controlled room temperature (15-30°C or 59-86°F) in a tight, light-resistant container. Keep away from moisture and excessive heat. Do not freeze the liquid formulation. Keep out of reach of children and pets. Discard any unused medication after completion of therapy or expiration date. Do not transfer tablets to other containers without proper labeling. The injectable form should be protected from light until administration.

Disclaimer

This information is provided for educational purposes only and does not constitute medical advice. Isoniazid is a prescription medication that must be used under appropriate medical supervision. Treatment decisions should be made by qualified healthcare professionals based on individual patient circumstances, susceptibility testing, and current treatment guidelines. Patients should not initiate or discontinue therapy without medical consultation. Healthcare providers should reference official prescribing information and current clinical guidelines for complete details on administration, monitoring, and adverse effect management.

Reviews

Clinical studies consistently demonstrate isoniazid’s efficacy, with meta-analyses showing 60-90% protection against tuberculosis development in latent infection and essential contribution to cure rates exceeding 95% in drug-susceptible active tuberculosis when used in appropriate combination regimens. The medication receives high marks from infectious disease specialists for its targeted action and established role in public health. Patient experiences vary, with most tolerating therapy well when properly monitored, though some report gastrointestinal discomfort or fatigue. The requirement for prolonged treatment remains a challenge for adherence, highlighting the importance of patient education and support systems.