Isoniazid 100mg Tablets
WARNING: Severe and sometimes deadly liver problems have happened with this drug. Call your doctor right away if you have signs of liver problems like dark urine, tiredness, decreased appetite, upset stomach or stomach pain, light-colored stools, throwing up, or yellow skin or eyes.The chance of liver problems is higher the older you are. The chance may also be raised by drinking alcohol every day, long-term liver problems, or injection drug use. The chance of liver problems may also be raised in females, mainly females who are Black or Hispanic or who have just had a baby. Most of the time, liver problems caused by this drug happen within the first 3 months of care, but they can happen at any time. Most of the time, liver function has gone back to normal but sometimes it has not. Blood work will need to be done before starting this drug and while taking it. If you have questions, talk with the doctor.If you have liver disease, talk with your doctor. This drug may not be right for you. @ COMMON USES: It is used to treat TB (tuberculosis). It is used to prevent TB (tuberculosis).
Drug Class
Antimycobacterial, Antitubercular agent
Pharmacologic Class
Mycolic acid synthesis inhibitor
Pregnancy Category
C
FDA Approved
Aug 1952
DEA Schedule
Not Controlled
Overview
What is this medicine?
Isoniazid is an antibiotic used to treat and prevent tuberculosis (TB), a serious bacterial infection that mainly affects the lungs. It works by stopping the growth of the bacteria that cause TB.
How to Use This Medicine
Taking Your Medication Correctly
To get the most benefit from your medication, follow your doctor's instructions carefully. Read all the information provided with your prescription and follow the instructions closely. Take your medication on an empty stomach, either 1 hour before or 2 hours after meals. Continue taking your medication as directed by your doctor or healthcare provider, even if you start to feel better.
Storing and Disposing of Your Medication
Store your medication at room temperature, away from light and moisture. Keep it in a dry place, such as a closet or drawer, and avoid storing it in the bathroom. Keep all medications out of the reach of children and pets. When you no longer need your medication or it has expired, dispose of it properly. Do not flush it down the toilet or pour it down the drain unless instructed to do so by your pharmacist. If you have questions about disposing of your medication, ask your pharmacist. You may also want to check if there are any drug take-back programs in your area.
What to Do If You Miss a Dose
If you miss a dose, take it as soon as you remember. However, if it's close to the time for your next dose, skip the missed dose and return to your regular schedule. Do not take two doses at the same time or take extra doses to make up for a missed one.
To get the most benefit from your medication, follow your doctor's instructions carefully. Read all the information provided with your prescription and follow the instructions closely. Take your medication on an empty stomach, either 1 hour before or 2 hours after meals. Continue taking your medication as directed by your doctor or healthcare provider, even if you start to feel better.
Storing and Disposing of Your Medication
Store your medication at room temperature, away from light and moisture. Keep it in a dry place, such as a closet or drawer, and avoid storing it in the bathroom. Keep all medications out of the reach of children and pets. When you no longer need your medication or it has expired, dispose of it properly. Do not flush it down the toilet or pour it down the drain unless instructed to do so by your pharmacist. If you have questions about disposing of your medication, ask your pharmacist. You may also want to check if there are any drug take-back programs in your area.
What to Do If You Miss a Dose
If you miss a dose, take it as soon as you remember. However, if it's close to the time for your next dose, skip the missed dose and return to your regular schedule. Do not take two doses at the same time or take extra doses to make up for a missed one.
Lifestyle & Tips
- Take Isoniazid on an empty stomach, at least 1 hour before or 2 hours after meals, for best absorption.
- Take Vitamin B6 (pyridoxine) daily as prescribed by your doctor to prevent nerve damage (peripheral neuropathy) caused by Isoniazid.
- Avoid or limit alcohol consumption during treatment, as it can significantly increase the risk of liver damage.
- Do not take antacids containing aluminum within 1 hour of taking Isoniazid, as they can reduce its absorption.
- Complete the full course of treatment as prescribed, even if you feel better, to prevent the TB bacteria from becoming resistant to the medication.
Available Forms & Alternatives
Dosing & Administration
Adult Dosing
Standard Dose:
Active TB: 5 mg/kg (max 300 mg) orally once daily OR 15 mg/kg (max 900 mg) orally 2-3 times per week. Latent TB: 300 mg orally once daily for 6-9 months.
Dose Range:
5 - 15 mg
Condition-Specific Dosing:
activeTBDaily:
5 mg/kg (max 300 mg) PO daily
activeTBIntermittent:
15 mg/kg (max 900 mg) PO 2-3 times/week
latentTB:
300 mg PO daily for 6-9 months
Pediatric Dosing
Neonatal:
Not established (use with extreme caution, consult specialist)
Infant:
Active TB: 10-15 mg/kg (max 300 mg) PO daily. Latent TB: 10-15 mg/kg (max 300 mg) PO daily for 6-9 months.
Child:
Active TB: 10-15 mg/kg (max 300 mg) PO daily OR 20-40 mg/kg (max 900 mg) PO 2-3 times/week. Latent TB: 10-15 mg/kg (max 300 mg) PO daily for 6-9 months.
Adolescent:
Active TB: 10-15 mg/kg (max 300 mg) PO daily OR 20-40 mg/kg (max 900 mg) PO 2-3 times/week. Latent TB: 10-15 mg/kg (max 300 mg) PO daily for 6-9 months.
Dose Adjustments
Renal Impairment:
Mild:
No adjustment usually needed.
Moderate:
No adjustment usually needed.
Severe:
CrCl < 10 mL/min: Consider dose reduction (e.g., 200 mg daily or 600 mg twice weekly).
Dialysis:
Administer after hemodialysis. Peritoneal dialysis: No dose adjustment needed.
Hepatic Impairment:
Mild:
Use with caution; monitor LFTs closely.
Moderate:
Use with caution; monitor LFTs closely. Consider dose reduction or discontinuation if signs of liver injury.
Severe:
Contraindicated in acute liver disease or history of INH-induced liver injury. Discontinue if signs of progressive liver damage.
Pharmacology
Mechanism of Action
Isoniazid is a prodrug that is activated by the mycobacterial catalase-peroxidase enzyme (KatG) to form isonicotinoyl-NAD. This complex inhibits the synthesis of mycolic acids, which are essential components of the mycobacterial cell wall, leading to cell death. It also inhibits other enzymes involved in nucleic acid and lipid metabolism.
Pharmacokinetics
Absorption:
Bioavailability:
90-100%
Tmax:
1-2 hours
FoodEffect:
Food (especially high-fat meals) significantly decreases the rate and extent of absorption. Should be taken on an empty stomach.
Distribution:
Vd:
0.6-0.8 L/kg
ProteinBinding:
<10%
CnssPenetration:
Yes (readily penetrates CSF, pleural fluid, ascitic fluid, and caseous lesions)
Elimination:
HalfLife:
1-4 hours (fast acetylators), 2-5 hours (slow acetylators)
Clearance:
Not available (highly variable due to acetylation polymorphism)
ExcretionRoute:
Renal (primarily as metabolites)
Unchanged:
10-30% (higher in slow acetylators)
Pharmacodynamics
OnsetOfAction:
Not well defined for clinical effect (bactericidal activity begins rapidly)
PeakEffect:
Not well defined for clinical effect (related to peak serum concentrations)
DurationOfAction:
Not well defined for clinical effect (dosing is daily or intermittent based on half-life and post-antibiotic effect)
Safety & Warnings
BLACK BOX WARNING
Severe and sometimes fatal hepatitis associated with isoniazid therapy may occur and may develop even after many months of treatment. The risk of developing hepatitis is age related and increases with concurrent use of alcohol. Patients should be monitored for symptoms of hepatitis, and liver enzymes should be measured at baseline and periodically during treatment, especially in high-risk patients. Isoniazid should be discontinued if signs or symptoms of hepatitis develop.
Side Effects
Urgent Side Effects: Seek Medical Help Right Away
Although rare, some people may experience severe and potentially life-threatening side effects while taking this medication. If you notice any of the following symptoms, contact your doctor or seek medical attention immediately:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin (with or without fever)
+ Wheezing
+ Tightness in the chest or throat
+ Difficulty breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Signs of high blood sugar, including:
+ Confusion
+ Drowsiness
+ Increased thirst or hunger
+ Frequent urination
+ Flushing
+ Rapid breathing
+ Fruity-smelling breath
Signs of lupus, such as:
+ Rash on the cheeks or other body parts
+ Easy sunburn
+ Muscle or joint pain
+ Chest pain or shortness of breath
+ Swelling in the arms or legs
Signs of pancreatitis (pancreas problem), including:
+ Severe stomach pain
+ Severe back pain
+ Severe upset stomach or vomiting
Signs of acidosis (too much acid in the blood), such as:
+ Confusion
+ Rapid breathing
+ Rapid heartbeat
+ Abnormal heartbeat
+ Severe stomach pain
+ Upset stomach or vomiting
+ Drowsiness
+ Shortness of breath
+ Feeling very tired or weak
Abnormal sensations, such as burning, numbness, or tingling
Seizures
Confusion or memory problems
Changes in vision
Mood changes
Fever, chills, or sore throat
Unexplained bruising or bleeding
Feeling very tired or weak
Joint pain or swelling
Enlarged breasts
Severe skin reactions, including:
+ Toxic epidermal necrolysis (TEN)
+ Red, swollen, blistered, or peeling skin
+ Red or irritated eyes
+ Sores in the mouth, throat, nose, eyes, genitals, or skin
+ Fever
+ Chills
+ Body aches
+ Shortness of breath
+ Swollen glands
Other Possible Side Effects
Like all medications, this drug can cause side effects. While many people may not experience any side effects or only minor ones, it's essential to discuss any concerns with your doctor. If you experience any of the following side effects or any other symptoms that bother you or do not go away, contact your doctor:
* Upset stomach or vomiting
This is not an exhaustive list of possible side effects. If you have questions or concerns, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
Although rare, some people may experience severe and potentially life-threatening side effects while taking this medication. If you notice any of the following symptoms, contact your doctor or seek medical attention immediately:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin (with or without fever)
+ Wheezing
+ Tightness in the chest or throat
+ Difficulty breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Signs of high blood sugar, including:
+ Confusion
+ Drowsiness
+ Increased thirst or hunger
+ Frequent urination
+ Flushing
+ Rapid breathing
+ Fruity-smelling breath
Signs of lupus, such as:
+ Rash on the cheeks or other body parts
+ Easy sunburn
+ Muscle or joint pain
+ Chest pain or shortness of breath
+ Swelling in the arms or legs
Signs of pancreatitis (pancreas problem), including:
+ Severe stomach pain
+ Severe back pain
+ Severe upset stomach or vomiting
Signs of acidosis (too much acid in the blood), such as:
+ Confusion
+ Rapid breathing
+ Rapid heartbeat
+ Abnormal heartbeat
+ Severe stomach pain
+ Upset stomach or vomiting
+ Drowsiness
+ Shortness of breath
+ Feeling very tired or weak
Abnormal sensations, such as burning, numbness, or tingling
Seizures
Confusion or memory problems
Changes in vision
Mood changes
Fever, chills, or sore throat
Unexplained bruising or bleeding
Feeling very tired or weak
Joint pain or swelling
Enlarged breasts
Severe skin reactions, including:
+ Toxic epidermal necrolysis (TEN)
+ Red, swollen, blistered, or peeling skin
+ Red or irritated eyes
+ Sores in the mouth, throat, nose, eyes, genitals, or skin
+ Fever
+ Chills
+ Body aches
+ Shortness of breath
+ Swollen glands
Other Possible Side Effects
Like all medications, this drug can cause side effects. While many people may not experience any side effects or only minor ones, it's essential to discuss any concerns with your doctor. If you experience any of the following side effects or any other symptoms that bother you or do not go away, contact your doctor:
* Upset stomach or vomiting
This is not an exhaustive list of possible side effects. If you have questions or concerns, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
Seek Immediate Medical Attention If You Experience:
- Signs of liver damage (hepatitis): unusual tiredness, weakness, loss of appetite, nausea, vomiting, dark urine, yellowing of the skin or eyes (jaundice), or pain in the upper right abdomen.
- Signs of nerve damage (peripheral neuropathy): numbness, tingling, burning, or pain in your hands or feet.
- Signs of optic neuritis: blurred vision, vision loss, or eye pain.
- Unexplained fever, rash, or joint pain (signs of a hypersensitivity reaction).
Before Using This Medicine
Before Taking This Medication: Important Information to Share with Your Doctor
It is essential to inform your doctor about the following:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Be sure to describe the allergic reaction and its symptoms.
If you have experienced severe side effects from taking isoniazid in the past, including liver problems, drug fever, chills, or arthritis.
* If you have a history of liver problems caused by other medications.
This list is not exhaustive, and it is crucial to discuss all your medications (prescription, over-the-counter, natural products, and vitamins) and health issues with your doctor and pharmacist. They will help determine if it is safe to take this medication with your existing treatments and health conditions. Never start, stop, or adjust the dosage of any medication without first consulting your doctor.
It is essential to inform your doctor about the following:
Any allergies you have, including allergies to this medication, its components, or other substances, such as foods or drugs. Be sure to describe the allergic reaction and its symptoms.
If you have experienced severe side effects from taking isoniazid in the past, including liver problems, drug fever, chills, or arthritis.
* If you have a history of liver problems caused by other medications.
This list is not exhaustive, and it is crucial to discuss all your medications (prescription, over-the-counter, natural products, and vitamins) and health issues with your doctor and pharmacist. They will help determine if it is safe to take this medication with your existing treatments and health conditions. Never start, stop, or adjust the dosage of any medication without first consulting your doctor.
Precautions & Cautions
It is essential to inform all your healthcare providers, including doctors, nurses, pharmacists, and dentists, that you are taking this medication. Before consuming alcohol, discuss the potential risks with your doctor. To minimize potential side effects, take vitamin B6 (pyridoxine) as directed by your doctor. If you have diabetes, it is crucial to closely monitor your blood sugar levels. Adhere to your doctor's recommendations for regular blood tests and eye exams. Certain foods and beverages, such as cheese and red wine, can cause a sudden and severe increase in blood pressure, which can be life-threatening. Consult your doctor to understand your risk and obtain a list of foods and drinks to avoid during treatment and for a specified period after stopping the medication. If you are pregnant, planning to become pregnant, or breastfeeding, inform your doctor to discuss the potential benefits and risks to you and your baby.
Overdose Information
Overdose Symptoms:
- Nausea, vomiting, dizziness, blurred vision, visual hallucinations
- Slurred speech, disorientation, hyperreflexia
- Severe metabolic acidosis, hyperglycemia
- Repetitive generalized seizures (often refractory to standard anticonvulsants)
- Coma, respiratory depression
What to Do:
Seek immediate medical attention. Call 911 or Poison Control at 1-800-222-1222. Treatment involves supportive care, administration of pyridoxine (Vitamin B6) intravenously in doses equal to the amount of isoniazid ingested (or empirically high doses for unknown ingestion), and management of seizures and metabolic acidosis.
Drug Interactions
Contraindicated Interactions
- Not typically contraindicated with other drugs, but severe interactions exist that may warrant avoidance.
Major Interactions
- Alcohol (increased risk of hepatotoxicity)
- Carbamazepine (increased levels of both INH and carbamazepine, leading to toxicity)
- Phenytoin (increased phenytoin levels, leading to toxicity)
- Disulfiram (may cause psychotic reactions, ataxia, or behavioral changes)
- Valproate (increased valproate levels and increased risk of hepatotoxicity)
- Acetaminophen (increased risk of hepatotoxicity with chronic use)
Moderate Interactions
- Antacids (aluminum-containing: decreased INH absorption; separate administration by at least 1 hour)
- Corticosteroids (may decrease INH efficacy)
- Ketoconazole/Itraconazole (altered antifungal levels, monitor)
- Theophylline (increased theophylline levels, monitor)
- Warfarin (increased anticoagulant effect, monitor INR)
- Cycloserine (increased CNS toxicity)
Minor Interactions
- Not typically listed for minor interactions with significant clinical impact.
Monitoring
Baseline Monitoring
Liver Function Tests (ALT, AST, bilirubin)
Rationale: To establish baseline liver status due to risk of hepatotoxicity.
Timing: Prior to initiation of therapy.
Renal Function (BUN, creatinine)
Rationale: To assess kidney function for potential dose adjustment in severe impairment.
Timing: Prior to initiation of therapy.
Complete Blood Count (CBC)
Rationale: To establish baseline and monitor for hematologic abnormalities (e.g., agranulocytosis, hemolytic anemia).
Timing: Prior to initiation of therapy.
Neurological Assessment
Rationale: To establish baseline for peripheral neuropathy symptoms.
Timing: Prior to initiation of therapy.
Ophthalmic Exam (if visual symptoms present)
Rationale: To establish baseline for optic neuritis.
Timing: Prior to initiation of therapy, especially if pre-existing visual issues.
Routine Monitoring
Liver Function Tests (ALT, AST)
Frequency: Monthly for the first 2-3 months, then periodically or if symptoms develop. More frequent in high-risk patients (e.g., >35 years old, alcohol users, pre-existing liver disease).
Target: Within normal limits
Action Threshold: Discontinue if ALT/AST > 3-5 times ULN with symptoms, or > 5 times ULN without symptoms.
Symptoms of Peripheral Neuropathy
Frequency: At each clinical visit.
Target: Absence of symptoms
Action Threshold: New or worsening numbness, tingling, burning, or weakness in extremities. Consider pyridoxine dose adjustment or INH discontinuation.
Symptoms of Hepatotoxicity
Frequency: At each clinical visit.
Target: Absence of symptoms
Action Threshold: New onset of unexplained fatigue, weakness, malaise, anorexia, nausea, vomiting, dark urine, yellow skin/eyes, or abdominal pain. Promptly evaluate LFTs.
Symptom Monitoring
- Peripheral neuropathy (numbness, tingling, burning, pain in hands/feet)
- Hepatotoxicity (unexplained fatigue, weakness, malaise, anorexia, nausea, vomiting, dark urine, yellow skin/eyes, abdominal pain)
- Optic neuritis (blurred vision, vision loss, eye pain)
- Fever, rash, joint pain (drug hypersensitivity reaction)
- Dizziness, lightheadedness
Special Patient Groups
Pregnancy
Category C. Isoniazid crosses the placenta. Use during pregnancy only if the potential benefit justifies the potential risk to the fetus. Pyridoxine supplementation is strongly recommended for pregnant women taking isoniazid to prevent maternal and fetal neuropathy.
Trimester-Specific Risks:
First Trimester:
Potential for teratogenicity not definitively established, but generally considered low risk. Pyridoxine supplementation is important.
Second Trimester:
Generally considered safe for continued use if indicated. Pyridoxine supplementation is important.
Third Trimester:
Generally considered safe for continued use if indicated. Pyridoxine supplementation is important.
Lactation
Isoniazid is excreted into breast milk. The American Academy of Pediatrics considers it compatible with breastfeeding. Monitor breastfed infants for signs of toxicity (e.g., drowsiness, poor feeding, fever, rash, jaundice). Pyridoxine supplementation for the infant may be considered, especially if the mother is a slow acetylator.
Infant Risk:
Low to moderate risk. Potential for hepatotoxicity or peripheral neuropathy in the infant, though rare. Benefits of breastfeeding generally outweigh risks if mother is on treatment.
Pediatric Use
Isoniazid is commonly used in pediatric patients for both active and latent TB. Dosing is weight-based. Children, especially infants, may be more susceptible to peripheral neuropathy, making pyridoxine supplementation crucial. Close monitoring for hepatotoxicity is also important.
Geriatric Use
Elderly patients, particularly those over 35 years of age, have an increased risk of developing isoniazid-induced hepatotoxicity. Close monitoring of liver function tests and symptoms is essential. Pre-existing liver disease or concurrent alcohol use further increases this risk.
Clinical Information
Clinical Pearls
- Always co-administer pyridoxine (Vitamin B6) with isoniazid to prevent peripheral neuropathy, especially in patients with risk factors such as malnutrition, diabetes, alcoholism, renal failure, or pregnancy.
- Monitor liver function tests (ALT, AST) monthly, particularly during the first 2-3 months of therapy and in patients at higher risk for hepatotoxicity (e.g., age >35, daily alcohol consumption, pre-existing liver disease).
- Educate patients thoroughly on the symptoms of hepatotoxicity and peripheral neuropathy and instruct them to report these immediately.
- Isoniazid should be taken on an empty stomach for optimal absorption.
- Compliance with the full course of therapy is critical to prevent treatment failure and the development of drug-resistant tuberculosis.
Alternative Therapies
- Rifampin
- Pyrazinamide
- Ethambutol
- Streptomycin
- Rifapentine
- Bedaquiline
- Delamanid
- Pretomanid
Cost & Coverage
Average Cost:
$10 - $30 per 30 tablets (100mg)
Generic Available:
Yes
Insurance Coverage:
Tier 1 (Generic)
General Drug Facts
If your symptoms or health issues persist or worsen, it is essential to contact your doctor promptly. To ensure safe and effective treatment, never share your medication with others or take someone else's medication. Some medications may come with an additional patient information leaflet, so it is a good idea to check with your pharmacist for more information. If you have any questions or concerns about your medication, do not hesitate to discuss them with your doctor, nurse, pharmacist, or other healthcare provider. In the event of a suspected overdose, immediately call your local poison control center or seek emergency medical attention. When reporting the incident, be prepared to provide details about the medication taken, the amount, and the time it occurred.