Isoniazid may cause severe and sometimes fatal liver damage. Tell your doctor if you have or have ever had liver disease or a history of alcoholism or injection drug use. Keep all appointments with your doctor and the laboratory. Your doctor will order certain lab tests to check your response to isoniazid.
If you experience any of the following symptoms, call your doctor immediately: excessive tiredness, weakness, lack of energy, loss of appetite, upset stomach, vomiting, dark yellow or brown urine, and yellowing of the skin or eyes.
Why is this medication prescribed?
Isoniazid is used alone or with other drugs to treat tuberculosis (TB) and to prevent it in people who have had contact with tuberculosis bacteria. It eliminates only active (growing) bacteria. Since the bacteria may exist in a resting (nongrowing) state for long periods, therapy with isoniazid (and other antituberculosis drugs) must be continued for a long time (usually 6 to 12 months).
This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.
How should this medicine be used?
Isoniazid comes as a tablet, capsule, and a syrup to take by mouth. It is also available as an extended release tablet. It usually is taken once a day, on an empty stomach, 1 hour before or 2 hours after meals. However, if isoniazid causes an upset stomach, it may be taken with food. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take isoniazid exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.
What special precautions should I follow?
Before taking isoniazid,
- tell your doctor and pharmacist if you are allergic to isoniazid or any other drugs.
- tell your doctor and pharmacist what prescription and nonprescription medications you are taking, especially acetaminophen (Tylenol), antacids, carbamazepine (Tegretol), disulfiram (Antabuse), ketoconazole (Nizoral), phenytoin (Dilantin), theophylline (Theobid, Theo-Dur), valproic acid (Depakene, Depakote), and vitamins.
- in addition to the conditions listed in the IMPORTANT WARNING section, tell your doctor if you have or have ever had kidney disease; diabetes; tingling, burning, and pain in the fingers or toes (peripheral neuropathy); or human immunodeficiency virus (HIV).
- tell your doctor if you are pregnant or plan to become pregnant. If you become pregnant while taking isoniazid, call your doctor.
- be aware that you should not drink alcoholic beverages while taking this drug.
What special dietary instructions should I follow?
Unless your doctor tells you otherwise, continue your normal diet.
What should I do if I forget a dose?
Take the missed dose as soon as you remember it. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not take a double dose to make up for a missed one.
What side effects can this medication cause?
Isoniazid may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:
- vision problems
If you experience any of the following symptoms or those listed in the IMPORTANT WARNING section, call your doctor immediately:
- eye pain
- numbness or tingling in the hands and feet
- skin rash
- swollen glands
- sore throat
- unusual bleeding or bruising
- stomach pains or tenderness
If you experience a serious side effect, you or your doctor may send a report to the Food and Drug Administration's (FDA) MedWatch Adverse Event Reporting program online [at http://www.fda.gov/Safety/MedWatch] or by phone [1-800-332-1088].
What should I know about storage and disposal of this medication?
Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom). Throw away any medication that is outdated or no longer needed. Talk to your pharmacist about the proper disposal of your medication.
In case of emergency/overdose
In case of overdose, call your local poison control center at 1-800-222-1222. If the victim has collapsed or is not breathing, call local emergency services at 911.
What other information should I know?
If you have diabetes, do not use Clinitest to test your urine for sugar because isoniazid can cause false results in this test.
Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.
It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.
Brand names of combination products
- IsonaRif® (containing Isoniazid, Rifampin)¶
- Rifamate® (containing Isoniazid, Rifampin)
- Rifater® (containing Isoniazid, Pyrazinamide, Rifampin)
¶ This branded product is no longer on the market. Generic alternatives may be available.