Isoniazid is used with other medications to treat active tuberculosis (TB) infections. It is also used alone to prevent active TB infections in people who may be infected with the bacteria (people with positive TB skin test). Isoniazid is an antibiotic and works by stopping the growth of bacteria.

This antibiotic treats only bacterial infections. It will not work for viral infections (such as common cold, flu). Using any antibiotic when it is not needed can cause it to not work for future infections.

Take this medication by mouth on an empty stomach (1 hour before or 2 hours after meals) as directed by your doctor. If you are using the liquid form of this medication, carefully measure the dose using a special measuring device/spoon. Do not use a household spoon because you may not get the correct dose.

If you also take antacids that contain aluminum, take this medication at least 1 hour before the antacid.

The dosage is based on your age, weight, medical condition, and response to treatment.

For the best effect, take this drug at evenly spaced times. If you are taking this medication daily, take it at the same time each day. If you are taking this medication on a weekly schedule, take it on the same day(s) and time each week. To help you remember, mark your calendar with a reminder.

Continue to take this medication (and other TB medications) until the full prescribed amount is finished, even if symptoms disappear. Stopping the medication too early or skipping doses may result in a return of the infection and cause the infection to be more difficult to treat (resistant).

Your doctor may also direct you to take vitamin B6 (pyridoxine) to help prevent certain side effects (such as nerve problems) from isoniazid. Follow your doctor's instructions carefully.

Isoniazid may interact with foods containing tyramine/histamine (such as cheese, red wine, certain types of fish). This interaction may cause increased blood pressure, flushing of the skin, headache, dizziness, or fast/pounding heartbeat. Tell your doctor right away if any of these symptoms occur. Your doctor may recommend that you follow a special diet while taking this medication. Consult your doctor for more details.

Tell your doctor if your condition lasts or gets worse.

See also Warning section.

Nausea/vomiting or stomach upset may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Remember that this medication has been prescribed because your doctor 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 right away if you have any serious side effects, including:

• numbness/tingling of arms/legs
• loss of coordination
• painful/swollen joints
• increased thirst/urination
• vision changes
• easy bruising/bleeding
• mental/mood changes (such as confusion, psychosis)
• trouble speaking
• seizures

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• fever that doesn't go away
• new or worsening lymph node swelling
• 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 or at www.fda.gov/medwatch.

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

See also Warning section.

Before taking isoniazid, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• previous severe reaction from isoniazid (such as liver disease)
• liver disease
• alcohol use
• HIV infection
• kidney disease
• diabetes
• numbness/tingling of arms/legs (peripheral neuropathy)
• recent childbirth

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Alcohol may increase the risk of liver disease. Avoid alcoholic beverages while using this medication.

This product may cause live bacterial vaccines (such as BCG vaccine) to not work well. Tell your health care professional that you are using this medication before having any immunizations/vaccinations.

Liquid forms of this medication may contain sugar. Caution is advised if you have diabetes or any other condition that requires you to limit/avoid sugar. Ask your doctor or pharmacist about using this product safely.

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

This product passes into breast milk but is unlikely to harm a nursing infant. Consult your doctor before breastfeeding.

See also How to Use section.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include:

• acetaminophen
• certain azole antifungals (itraconazole, ketoconazole)
• disulfiram
• MAO inhibitors (isocarboxazid, linezolid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, tranylcypromine)
• phenytoin
• SSRI antidepressants (such as fluoxetine, sertraline)
• valproic acid

This medication may interfere with certain lab tests (such as urine glucose tests), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: vision changes, slow/shallow breathing, severe drowsiness.

Do not share this medication with others.

Lab and/or medical tests (such as sputum test, kidney/liver function, complete blood count) should be done while you are taking this medication. Keep all medical and lab appointments. Consult your doctor for more details.

It is important to take each dose of this medication as scheduled. If you are taking this medication daily and miss a dose, take it as soon as you remember. If it is near the time of the next dose, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up.

If you are taking this drug on a weekly schedule and miss a dose, take it as soon as you remember. If you don't remember until the next day or later, ask your doctor or pharmacist right away for a new dosing schedule. Do not double the dose to catch up.

Store at room temperature away from light and moisture. Do not store in the bathroom. Keep all medications 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.