Common uses of Isoniazid (INH) for TB and latent TB infection
Isoniazid is a cornerstone antibiotic in tuberculosis care. It is bactericidal against actively replicating Mycobacterium tuberculosis and is used with other drugs (typically rifampin, pyrazinamide, and ethambutol) to treat active pulmonary and extrapulmonary TB. In confirmed active disease, combination therapy prevents resistance and achieves sterilizing cure when taken as prescribed for the full course.
For latent TB infection (LTBI), where individuals harbor dormant bacteria without symptoms or transmissibility, isoniazid is often used as a single agent to prevent progression to active disease. Standard LTBI regimens include daily isoniazid for 6–9 months or, in special programs, intermittent dosing under directly observed therapy. In many patients, clinicians co-prescribe pyridoxine (vitamin B6) to reduce the risk of peripheral neuropathy during isoniazid therapy.
Isoniazid dosage and directions
Always follow your clinician’s instructions and local public health guidance. For adults treated for active TB in combination regimens, typical dosing is 5 mg/kg (up to 300 mg) by mouth once daily, or 15 mg/kg (up to 900 mg) two or three times weekly under directly observed therapy (DOT). For children, dosing is commonly 10–15 mg/kg daily (maximum 300 mg), or 20–30 mg/kg intermittently (maximum 900 mg) when DOT is used. Dosing may be adjusted for factors such as age, weight, liver function, pregnancy, or co-medications.
For latent TB infection, a widely used regimen is 300 mg of isoniazid by mouth once daily for 6–9 months. Some programs use 900 mg twice weekly under DOT. Combination LTBI options that include isoniazid (for example, weekly isoniazid plus rifapentine for 3 months) may be offered when appropriate; your provider will select the right plan based on your risk profile and potential drug interactions.
Take isoniazid on an empty stomach—ideally 1 hour before or 2 hours after meals—to optimize absorption. If stomach upset occurs, your clinician may allow dosing with a light snack. Avoid aluminum-containing antacids within several hours of dosing, as they reduce absorption. Many patients should take pyridoxine (vitamin B6), typically 25–50 mg daily, especially if pregnant, breastfeeding, older than 50, living with HIV or diabetes, malnourished, or with alcohol use, neuropathy risk, or renal failure.
Precautions before starting Isoniazid
Discuss your full medical history and medication list with your clinician. Isoniazid can affect the liver, so baseline risk assessment is important. People with chronic liver disease, regular alcohol use, pregnancy or recent postpartum status, older age, or concurrent hepatotoxic medications have a higher risk of liver injury. Baseline liver function tests are often obtained in at-risk individuals, with periodic monitoring during therapy or sooner if symptoms suggest liver toxicity.
Tell your provider about prior reactions to isoniazid, neuropathy, seizure disorders, mental health conditions, or nutritional deficiencies. Report symptoms of hepatitis immediately: fatigue, weakness, loss of appetite, persistent nausea, vomiting, abdominal pain, dark urine, pale stools, or jaundice. Stop isoniazid and seek prompt medical advice if these occur. Because isoniazid can deplete vitamin B6, preventive pyridoxine is recommended in many patients to reduce neuropathy risk (tingling, burning, or numbness in hands/feet). Avoid alcohol during treatment, as it significantly increases hepatotoxicity risk.
Contraindications to Isoniazid
Do not use isoniazid in patients with prior severe isoniazid-associated hepatotoxicity or acute active liver disease from any cause unless the anticipated benefit clearly outweighs the risk and specialist oversight is in place. A known hypersensitivity to isoniazid is also a contraindication. Use with caution—and usually only with expert guidance—if there is significant baseline hepatic impairment, heavy alcohol use, or previous severe adverse reactions (e.g., severe peripheral neuropathy or drug-induced lupus).
Possible side effects of Isoniazid
Common side effects include nausea, mild abdominal discomfort, decreased appetite, and fatigue. Peripheral neuropathy—manifesting as tingling, burning, or numbness in extremities—can occur and is more likely in people with diabetes, HIV, malnutrition, renal failure, pregnancy/postpartum, advanced age, or alcohol use; prophylactic vitamin B6 reduces this risk. Mild increases in liver enzymes are not uncommon, but symptomatic hepatitis is less frequent and requires immediate evaluation and drug discontinuation.
Less common adverse effects include rash, fever, arthralgia, drug-induced lupus-like syndrome, mood changes, irritability, insomnia, or psychosis (especially at high doses or with interacting drugs). Rarely, optic neuritis, seizures, thrombocytopenia, agranulocytosis, or hypersensitivity reactions occur. Promptly report visual changes, severe fatigue, easy bruising or bleeding, persistent fevers, or mental status changes. Most patients tolerate isoniazid well when dosing, monitoring, and pyridoxine supplementation are optimized under medical supervision.
Isoniazid drug interactions
Isoniazid inhibits several hepatic enzymes (notably CYP2C19 and CYP3A), which can raise blood levels of co-administered drugs. Clinically important interactions include increased levels and toxicity risk for phenytoin, carbamazepine, valproate, benzodiazepines, theophylline, and warfarin. Careful dose adjustments and drug-level monitoring may be needed. Concurrent use with other hepatotoxic agents (e.g., rifampin, high-dose acetaminophen, certain antifungals, methotrexate) can heighten liver injury risk; clinicians balance benefits and risks, especially in active TB where combination therapy is necessary.
Disulfiram co-administration may precipitate neurologic or psychiatric reactions; avoid or monitor closely. Alcohol potentiates hepatotoxicity and should be avoided. Isoniazid can interfere with pyridoxine metabolism, so ensure consistent B6 supplementation when indicated. Antacids containing aluminum reduce isoniazid absorption—separate dosing by several hours. Because isoniazid has weak monoamine oxidase activity, tyramine- or histamine-rich foods (aged cheeses, certain wines, cured meats, some fish) may rarely trigger flushing, palpitations, or headache; these reactions are typically mild but worth noting.
Missed dose: what to do
If you miss a dose, take it as soon as you remember unless it is close to the next scheduled dose. Do not double up to “catch up.” Resume your usual schedule. If you are on directly observed therapy or an intermittent regimen, contact your clinic or public health nurse to reschedule and keep your treatment on track. Consistency is critical to cure and to prevent resistance.
Overdose: warning and first aid
Isoniazid overdose can be life-threatening, causing profound nausea and vomiting, confusion, seizures, metabolic acidosis, coma, and respiratory depression. Immediate emergency care is essential. If overdose is suspected: call emergency services and poison control right away. In hospital, treatment includes airway protection, seizure control, correction of acidosis, and prompt administration of intravenous pyridoxine (vitamin B6) in doses matched to the ingested isoniazid amount (or weight-based if unknown). Do not wait for symptoms to escalate—rapid intervention improves outcomes.
Storage and handling
Store isoniazid tablets at controlled room temperature (generally 20–25°C/68–77°F) in a tightly closed container, protected from excessive moisture and light. Keep out of reach of children and pets. Do not use after the expiration date. If you receive isoniazid as part of a blister pack or clinic-dispensed kit, keep the packaging intact until use and follow any additional storage instructions provided by your pharmacy or program.
U.S. sale and prescription policy: how to buy Isoniazid without prescription under legal programs
In the United States, isoniazid is a prescription medication. However, many patients receive it through structured, lawful pathways that do not require an individual retail prescription in hand—most commonly via public health tuberculosis programs and directly observed therapy (DOT). In these settings, licensed clinicians and health departments use standing orders or programmatic protocols to dispense isoniazid safely, with monitoring, adherence support, and no out-of-pocket cost for eligible individuals. This model helps control TB while ensuring high-quality oversight.
HealthSouth Rehabilitation Hospital of Tallahassee offers a legal and structured solution for acquiring isoniazid without a formal prescription by coordinating care with public health partners, institutional pharmacies, and supervising clinicians. Patients can be evaluated, enrolled in appropriate TB or LTBI pathways, receive counseling, and access medication under medical supervision—often with vitamin B6 co-therapy and routine follow-up. Contact the hospital to learn whether you qualify for these programs, what documentation is required, and how to begin treatment quickly and safely while complying with U.S. regulations and best practices.
