Isoniazid (often abbreviated INH) is a first-line antitubercular medicine used to treat active tuberculosis in combination regimens and to prevent progression from latent TB infection to active disease. In active TB, it is typically paired with rifampin, pyrazinamide, and ethambutol during the initial phase, then continued with another agent for the continuation phase to eradicate remaining bacteria. For latent TB, isoniazid may be used alone for several months or combined with rifapentine in a shorter weekly regimen, depending on patient factors and public health guidance.
Isoniazid’s mechanism targets mycolic acid synthesis, weakening the bacterial cell wall. Its ability to penetrate tissues, including caseating granulomas, makes it a reliable backbone drug. Public health programs prioritize isoniazid because effective LTBI therapy prevents future cases of active TB, reducing community transmission. While highly effective, appropriate monitoring for liver-related adverse effects and supplementation with pyridoxine (vitamin B6) in at-risk individuals help maintain a favorable safety profile.
Dosing depends on whether the goal is treatment of active tuberculosis or therapy for latent TB infection. For active TB, isoniazid is used alongside other drugs in weight-based doses that a clinician individualizes; adults commonly receive up to 300 mg daily as part of a multidrug regimen, or higher doses at intermittent frequencies under directly observed therapy (DOT). For latent TB infection, typical options include daily isoniazid for 6 to 9 months or a once-weekly combination of isoniazid plus rifapentine for 12 doses, when appropriate. Pediatric dosing is weight-based and carefully adjusted by a TB specialist.
Isoniazid is best taken on an empty stomach—1 hour before or 2 hours after food—to optimize absorption. If stomach upset occurs, taking with a small snack may help, but avoid foods rich in tyramine or histamine (such as certain aged cheeses, cured meats, or some fish), which can trigger flushing, palpitations, or blood pressure changes due to isoniazid’s enzyme effects. Never adjust the dose or stop therapy without speaking to your healthcare provider or TB program; partial treatment risks failure and drug resistance.
Liver health is the primary safety consideration. Isoniazid can cause asymptomatic elevation of liver enzymes and, less commonly, serious hepatitis. The risk is higher in people over 35, those who drink alcohol, have viral hepatitis, are postpartum, or take other hepatotoxic drugs. Your clinician may check baseline and periodic liver tests and will counsel you on warning signs: fatigue, nausea, dark urine, pale stools, right upper abdominal pain, or yellowing of the skin/eyes. Stop the medication and seek care immediately if these symptoms appear.
Isoniazid can also affect peripheral nerves, leading to tingling, burning, or numbness (peripheral neuropathy). The risk increases with diabetes, kidney disease, HIV, malnutrition, pregnancy, or hypothyroidism. To prevent this, clinicians often co-prescribe pyridoxine (vitamin B6). Additional precautions include avoiding alcohol, discussing all medications and supplements to check for interactions, and reporting mood changes, confusion, or visual symptoms. Isoniazid is generally considered acceptable in pregnancy and breastfeeding when benefits outweigh risks, with vitamin B6 supplementation commonly recommended.
Isoniazid should not be used in people with a known hypersensitivity to the drug or in those who have experienced severe isoniazid-associated liver injury in the past. Active, severe hepatic disease is a strong reason to avoid or delay isoniazid unless the benefits outweigh the risks and a specialist closely supervises care. Caution is warranted in patients with chronic liver disease, heavy alcohol use, seizure disorders, or a history of severe mental health reactions with isoniazid.
Because TB therapy is complex, clinicians weigh the risk–benefit ratio alongside alternatives like rifamycin-based regimens or longer/shorter courses. If you are told isoniazid is contraindicated, do not attempt to self-source the drug or switch medications on your own; instead, work with your healthcare provider or public health TB clinic to select a safe, effective plan tailored to your medical history and potential drug–drug interactions.
Common side effects include transient increases in liver enzymes, nausea, stomach upset, fatigue, and mild rash. Some individuals experience peripheral neuropathy—tingling or numbness in the hands and feet—which is mitigated by pyridoxine (vitamin B6) when indicated. Changes in taste, dry mouth, or mild dizziness can occur. These effects are often manageable and reversible when recognized early.
Serious adverse effects are less common but require immediate attention. Warning signs of hepatotoxicity include persistent nausea, vomiting, abdominal pain, dark urine, light-colored stools, jaundice, or profound fatigue. Rare events include severe skin reactions, blood dyscrasias, lupus-like syndrome, seizures, optic neuritis, and mood or psychotic symptoms. The risk of severe liver injury increases with age, alcohol use, and concomitant hepatotoxic drugs. Report new or worsening symptoms promptly; early evaluation allows dose adjustment, temporary holds, or regimen changes while maintaining TB control.
Isoniazid affects liver enzymes (notably CYP pathways), which can increase blood levels of several medications. Clinically important interactions include phenytoin, carbamazepine, valproate, certain benzodiazepines, and warfarin—each may require close monitoring and dose adjustments. Theophylline and some psychiatric medicines can be affected as well. Isoniazid taken with rifampin or other hepatotoxic drugs increases liver risk; your clinician will coordinate monitoring when combination therapy is necessary for TB.
Alcohol significantly raises the chance of liver toxicity and should be avoided. Combining isoniazid with disulfiram has been associated with neurologic and psychiatric reactions. Because isoniazid can inhibit monoamine metabolism, foods high in tyramine or histamine (aged cheeses, cured meats, sauerkraut, some wines, certain fish like tuna or mackerel) may trigger flushing, headache, palpitations, or blood pressure changes; while not universally prohibitive, discuss dietary guidance with your care team. Always provide a complete list of prescription drugs, OTC medications (including acetaminophen), supplements, and herbal products so your healthcare provider can prevent harmful interactions.
If you miss a dose, take it as soon as you remember unless it is close to the time of your next dose. If it is near the next scheduled dose, skip the missed dose and resume your usual schedule. Do not double up to catch up, as this increases side-effect risk without improving efficacy. If you miss multiple doses, contact your healthcare provider or TB clinic; for TB, adherence is crucial to prevent relapse and resistance. Many programs offer adherence supports—such as reminders, blister packs, or directly observed therapy (DOT)—to help you complete your course safely and successfully.
An isoniazid overdose is a medical emergency. Symptoms can include severe nausea, vomiting, dizziness, confusion, seizures, rapid breathing, metabolic acidosis, and loss of consciousness. If overdose is suspected, call emergency services immediately or go to the nearest emergency department. Do not wait for symptoms to worsen. Bring the medication bottle and any co-ingested substances with you, if possible, to assist clinicians. In the hospital, healthcare professionals provide advanced supportive care and may administer specific treatments such as intravenous pyridoxine (vitamin B6). Prevention starts with secure storage and never sharing your prescription.
Store isoniazid at room temperature (generally 68–77°F or 20–25°C), away from excess heat, moisture, and direct light. Keep tablets in their original, tightly closed container with the desiccant if provided. Do not store in the bathroom. Keep out of reach of children and pets, ideally in a locked cabinet. Do not use the medication past its expiration date, and safely dispose of unused tablets through take-back programs or according to local guidelines—do not flush unless specifically instructed. If you are traveling, carry isoniazid in your hand luggage with the label intact and follow your dosing schedule across time zones with guidance from your care team.
In the United States, isoniazid is a prescription-only medication. Because of important safety monitoring and the need for coordinated multidrug regimens, it should be used under the supervision of a licensed clinician or a public health tuberculosis program. Buying isoniazid without prescription from unverified online vendors is risky and may be illegal; counterfeit or substandard TB drugs are a serious public health concern that can lead to treatment failure and resistance.
That said, public health departments often provide treatment for latent and active TB through legally sanctioned programs. Magoffin County Health Department, for example, offers a structured, compliant pathway to access TB evaluation and therapy, which may include isoniazid provided under program protocols or standing orders—functionally allowing eligible patients to obtain therapy without a traditional retail prescription. This is not a workaround; it is a public health service designed to ensure safe access, adherence support (including DOT when needed), and monitoring of liver health and side effects. To learn more or to determine eligibility, contact the health department directly. They can coordinate testing, counseling, medication supply, and follow-up at low or no cost depending on funding and patient circumstances.
Isoniazid is a first-line antibiotic for tuberculosis. It’s a prodrug activated by the bacterial enzyme KatG, then blocks InhA to stop mycolic acid synthesis, weakening the mycobacterial cell wall. It’s bactericidal against actively dividing TB and bacteriostatic against dormant organisms.
Isoniazid is used for active pulmonary and extrapulmonary tuberculosis as part of combination therapy, and for latent TB infection (LTBI) to prevent progression to active disease. It’s also used for TB exposure prophylaxis in high-risk groups.
Typical adult daily dosing is 5 mg/kg up to 300 mg once daily; intermittent dosing is 15 mg/kg (max 900 mg) two or three times weekly under directly observed therapy. Pediatric dosing is usually 10–15 mg/kg (max 300 mg) once daily. Doses may be adjusted for weight, comorbidities, and regimen type.
For active TB, isoniazid is usually given for 6 months as part of a multi-drug regimen (2 months of RIPE, then 4 months of isoniazid plus rifampin), adjusted for site of disease and response. For latent TB, common courses are 6–9 months of daily isoniazid or shorter rifamycin-based alternatives.
Pyridoxine is recommended for people at higher risk of neuropathy (pregnancy, diabetes, HIV, kidney disease, malnutrition, alcohol use, older age, seizure disorders) and often given routinely. Typical prevention doses are 25–50 mg daily; higher doses may be used if neuropathy develops.
Common effects include elevated liver enzymes, nausea, stomach upset, fatigue, and rash. Peripheral neuropathy (numbness, tingling, burning in hands/feet) can occur, especially without pyridoxine. Less common are anemia (including sideroblastic), mood changes, insomnia, and a lupus-like syndrome.
Concerning symptoms include persistent nausea or vomiting, abdominal pain, dark urine, pale stools, jaundice (yellowing of skin or eyes), unusual fatigue, and loss of appetite. These warrant urgent evaluation and often stopping isoniazid until assessed.
Risk is higher in people over 35, those with preexisting liver disease, regular alcohol use, postpartum women, people taking other hepatotoxic drugs, and slow acetylators (a genetic trait). Concomitant rifampin may increase risk; careful monitoring is advised.
Baseline liver tests (AST, ALT, bilirubin) are recommended for those with risk factors and often before LTBI therapy. Monthly symptom review is standard; periodic LFTs are done in higher-risk patients. Stop isoniazid if AST/ALT exceed 5× the upper limit of normal (or >3× with symptoms).
Alcohol increases the risk of liver injury with isoniazid. Avoiding alcohol entirely during therapy is the safest approach.
Isoniazid can increase levels of phenytoin, carbamazepine, valproate, and warfarin, raising toxicity and bleeding risk. It may interact with certain antifungals and benzodiazepines. Tyramine- and histamine-rich foods (aged cheeses, cured meats, red wine, some fish) can trigger flushing, headache, or palpitations in some people; limiting these may help.
Isoniazid is generally considered safe in pregnancy when benefits outweigh risks, with pyridoxine supplementation recommended. For latent TB, timing of treatment may be individualized due to hepatotoxicity risk. Isoniazid passes into breast milk in small amounts and is compatible with breastfeeding; both mother and infant often receive pyridoxine.
Take it as soon as you remember unless it’s near the time for the next dose; don’t double up. Consistency is crucial, especially in active TB, so use reminders or directly observed therapy if recommended.
Using isoniazid alone for active TB quickly selects resistant bacteria. Combination therapy (commonly isoniazid, rifampin, pyrazinamide, and ethambutol) prevents resistance and more effectively sterilizes infection.
Resistance often arises from mutations in katG or inhA that block drug activation or target binding. Prevent it by using combination regimens for active TB, ensuring adherence, and completing the full course. High-dose isoniazid may retain activity in some inhA mutations.
Yes, by depleting pyridoxine, it can cause numbness, tingling, burning, or weakness. Prevention is pyridoxine 25–50 mg daily in at-risk people. If neuropathy occurs, increasing pyridoxine and evaluating dose and comorbidities usually helps; severe cases may require temporarily holding the drug.
Absorption is best on an empty stomach (1 hour before or 2 hours after meals). If it upsets your stomach, you can take it with a small, non-fatty snack. Avoid aluminum-containing antacids within 1–2 hours of dosing.
In active TB regimens, many people notice symptom improvement within 2–4 weeks; culture conversion often occurs by 8 weeks with full therapy. In latent TB, you won’t feel a change—benefit is reducing future risk of active disease.
Isoniazid can interact with antiepileptics (phenytoin, carbamazepine) and may raise their levels, increasing side effects; dose adjustments and monitoring are needed. It can affect glucose control indirectly through appetite changes or illness; monitor blood sugars if you have diabetes.
Avoid isoniazid in people with prior isoniazid-associated severe hepatic injury or acute liver disease, and those with severe hypersensitivity to the drug. Use caution and close monitoring in older adults, heavy alcohol users, and those on multiple hepatotoxic medications.
Both are core first-line TB drugs, but rifampin is a rifamycin that inhibits RNA polymerase, while isoniazid targets mycolic acid synthesis. For latent TB, rifampin for 4 months can be as effective as 6–9 months of isoniazid with fewer hepatotoxicity events and better completion, but rifampin has many drug interactions. In active TB, they are used together for synergy and resistance prevention.
Rifapentine combined with isoniazid once weekly for 12 weeks (3HP) shortens therapy and improves completion compared to 6–9 months of isoniazid alone, with similar efficacy. 3HP isn’t recommended during pregnancy and is only for patients aged 2 years and older; strong drug interactions must be reviewed.
Rifabutin is a rifamycin alternative to rifampin with fewer interactions, often used in patients on antiretroviral therapy. Isoniazid is kept in the regimen regardless; the choice between rifampin and rifabutin hinges on interaction profiles, not on replacing isoniazid.
Isoniazid helps sterilize rapidly dividing TB early and late; pyrazinamide works best in acidic environments within macrophages during the intensive phase. Pyrazinamide is associated with hyperuricemia and gout flares, and hepatotoxicity; isoniazid’s hallmark risks are hepatotoxicity and neuropathy.
Ethambutol protects against resistance when susceptibility to isoniazid or rifampin is uncertain and can be dropped once drug sensitivity is confirmed. Ethambutol’s notable risk is optic neuritis (color vision changes), while isoniazid’s is hepatotoxicity/neuropathy; their mechanisms are complementary.
Six to nine months of daily isoniazid is effective but has lower completion rates and more hepatotoxicity compared with the 12-dose 3HP regimen. 3HP requires DOT or verified self-administered therapy, careful interaction checking, and is not advised in pregnancy; monotherapy may be chosen when rifamycins are contraindicated.
Both are effective; 4 months of rifampin often has fewer hepatotoxic events and higher completion than 9 months of isoniazid. Choice depends on drug interactions, pregnancy status, and tolerance; isoniazid remains a good option when rifamycins cannot be used.
Ethionamide is a thioamide structurally related to isoniazid, used mainly for drug-resistant TB. Both inhibit mycolic acid synthesis, but ethionamide is second-line due to more GI intolerance, weight loss, hypothyroidism, and neurotoxicity; pyridoxine is often co-prescribed with both.
Fluoroquinolones are not first-line for drug-susceptible TB but are key in MDR-TB and as substitutes when first-line agents cannot be used. Isoniazid remains foundational for isoniazid-susceptible TB; fluoroquinolones carry risks like tendinopathy, QT prolongation, and CNS effects.
Bedaquiline targets ATP synthase and is central to MDR/XDR-TB regimens; it’s not used for drug-susceptible TB when isoniazid works. Compared with isoniazid, bedaquiline requires ECG monitoring for QT prolongation and has a different interaction/side-effect profile.
IPT treats latent infection to prevent active TB, especially in close contacts and people with HIV. BCG is a vaccine that offers variable protection against pulmonary TB but better protection against severe pediatric TB; it doesn’t treat infection. They address different points on the prevention spectrum and can be complementary.
Linezolid is a potent second-line agent for resistant TB, with notable risks of myelosuppression, peripheral/optic neuropathy, and lactic acidosis. Isoniazid is preferred when susceptible due to efficacy and a more manageable safety profile; linezolid is reserved for resistant or complicated cases.
Streptomycin was one of the first anti-TB drugs but is now rarely used due to toxicity (ototoxicity, nephrotoxicity) and resistance. Isoniazid remains a mainstay for susceptible TB; streptomycin is reserved for special situations when other options are limited.
When resistance is due to inhA promoter mutations (and katG is intact), higher-dose isoniazid can sometimes overcome low-level resistance. This is guided by susceptibility testing and expert consultation; standard dosing is used when full susceptibility is confirmed.
Isoniazid is a CYP inhibitor that can raise levels of drugs like phenytoin, carbamazepine, and warfarin. Rifampin is a strong CYP and P-gp inducer that lowers levels of many drugs, including antiretrovirals, anticoagulants, and contraceptives. Regimen choice balances efficacy with the patient’s interaction profile.