Rarely, isoniazid has caused a severe (sometimes fatal) liver problem (hepatitis). Hepatitis can develop at any time while you are taking this medication. Immediately tell your doctor if you develop symptoms of liver problems (persistent nausea, vomiting, weakness, tiredness, stomach/abdominal pain, dark urine, yellowing eyes/skin). The risk of hepatitis increases with age (35 and older), persistent liver problems, and the use of alcohol and/or illegal injection drugs. Do not use alcohol or illegal injection drugs while taking this medication.

You will need certain blood tests (liver function tests) regularly to check for side effects. Keep all medical and laboratory appointments.

This medication is used with other medications to treat active tuberculosis (TB) infections or alone to prevent those who have a positive TB test from developing symptoms of TB. Isoniazid belongs to a class of drugs known as antibiotics that are active against tuberculosis.

Take this medication by mouth without food, usually once weekly to once daily, or as directed by your doctor. Depending on the type of TB you have, you may take isoniazid for 2-9 months.

Dosage is based on your age, weight, medical condition, and response to treatment. If you are taking a liquid form of this medication, carefully measure your prescribed dose using a medication-measuring device or spoon. Do not use a household spoon because you may not get the correct dose.

It is very important to continue taking this medication (and other TB medications) exactly as prescribed by your doctor.

This medication works best when the amount of drug in your body is kept at a constant level. Therefore, take this drug at evenly spaced intervals. To help you remember, take it on the same day(s) of the week or at the same time(s) each day. If you are taking this medication several times a week, it may help to mark your calendar with a reminder.

Do not take more or less of this drug than prescribed or stop taking it (or other TB medicines) even for a short time unless directed to do so by your doctor. Skipping or changing your dose without approval from your doctor may cause the amount of TB bacteria to increase, make the infection more difficult to treat (resistant), or worsen side effects. If TB becomes resistant to this medication, it might also be resistant to other TB medications.

Your doctor may also direct you to take vitamin B6 (pyridoxine) to help prevent certain side effects (nerve problems) from isoniazid.

Inform your doctor if your condition persists or worsens.

See also Warning section.

Tingling/numbness in the hands/feet or stomach pain may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.

Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor immediately if any of these unlikely but serious side effects occur: increase in the amount of urine, breast enlargement in males.

Tell your doctor immediately if any of these rare but very serious side effects occur: vision changes, confusion, seizures, sudden weakness, pale skin, fast breathing, easy bruising/bleeding, new fever, persistent sore throat, chills.

This medication may rarely cause a serious immune system problem (systemic lupus erythematosus). Tell your doctor immediately if you experience any of these unlikely but serious side effects: unusual tiredness, joint/muscle aches, unusual fever, butterfly-shaped facial rash, swollen glands, bloody/pink urine, swelling of the feet/ankles.

Your doctor may need to order special tests if you experience these symptoms. Keep all laboratory and medical appointments.

A very serious allergic reaction to this drug is rare. However, seek immediate medical attention if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US -

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before taking isoniazid, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies.

This medication should not be used if you have certain medical conditions. Before using this medicine, consult your doctor or pharmacist if you have: severe and persistent liver disease, alcoholism, new liver disease not yet diagnosed.

Before using this medication, tell your doctor or pharmacist your medical history, especially of: persistent liver problems, HIV infection, cancer, kidney problems, certain nerve problems (peripheral neuropathy), diabetes, seizures, recent childbirth (in women of African/Latina heritage), illegal injection drug use.

If you have diabetes, this product may make it harder to control your blood sugar levels. Check your blood sugar levels regularly as directed by your doctor. Tell your doctor immediately if you have symptoms of high blood sugar such as increased thirst and urination. Your anti-diabetic medication or diet may need to be adjusted.

Liquid forms of this medication may contain sugar or aspartame. Caution is advised if you have diabetes, phenylketonuria (PKU), or any other condition that requires you to limit/avoid these substances in your diet. Ask your doctor or pharmacist about using this product safely.

Caution is advised when using this drug in the elderly because they may be more sensitive to the effects of the drug, especially the liver effects.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

This medication passes into breast milk. Though there have been no reports of harm to nursing infants, consult your doctor before breast-feeding.

Your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor or pharmacist first.

This drug should not be used with the following medications because very serious interactions may occur: ketoconazole, itraconazole.

If you are currently using any of these medications listed above, tell your doctor or pharmacist before starting isoniazid.

Before using this medication, tell your doctor or pharmacist of all prescription and nonprescription/herbal products you may use, especially of: acetaminophen, carbamazepine, phenytoin, theophylline, valproic acid.

This document does not contain all possible interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share the list with your doctor and pharmacist.

If overdose is suspected, contact your local poison control center or emergency room immediately. US residents should call the US National Poison Hotline at 1-800-222-1222. Canada residents should call a provincial poison control center. Symptoms of overdose may include: nausea/vomiting, slurring of speech, blurred vision, hallucinations, trouble breathing, fainting, inability to wake up (coma), severe/persistent seizures.

Do not share this medication with others.

Keep all medical and laboratory appointments.

Laboratory and/or medical tests (e.g., TB cultures, liver function tests) should be performed periodically to monitor your progress or check for side effects. Consult your doctor for more details.

If you miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose and resume your usual dosing schedule. Do not double the dose to catch up.

Store at room temperature between 59-86 degrees F (15-30 degrees C) away from light and moisture. Do not store in the bathroom. Keep all medicines away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company for more details about how to safely discard your product.

Information last revised October 2008 Copyright(c) 2008 First DataBank, Inc.