Isoniazid Overview

Reviewed: June 8, 2012

Isoniazid is a prescription medication used in the prevention and treatment of tuberculosis. Isoniazid belongs to a group of drugs called antitubercular agents. It is believed to work by preventing the bacteria from multiplying in the body.

This medication comes in tablet, capsule and syrup forms. It usually is taken once a day, on an empty stomach, 1 hour before or 2 hours after meals. 
Common side effects of Isoniazid include tingling of hands and feet, nausea, vomiting, and diarrhea. 
Do not drink alcohol while taking isoniazid. Alcohol may increase the risk of liver damage during isoniazid treatment.

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Isoniazid Cautionary Labels


Uses of Isoniazid

Isoniazid is used to prevent or treat tuberculosis, alone or in combination with other medicines.

This medication may be prescribed for other uses. Ask your doctor or pharmacist for more information.

Isoniazid Brand Names

Isoniazid may be found in some form under the following brand names:

Isoniazid Drug Class

Isoniazid is part of the drug class:

Side Effects of Isoniazid

If you experience any of the following symptoms of liver damage, call your doctor immediately:

  • excessive tiredness/weakness
  • lack of energy
  • loss of appetite
  • upset stomach, vomiting
  • dark urine
  • yellowing of the skin or eyes

Tell your doctor immediately if you have the following symptoms:

  • numbness or tingling in the hands and feet
  • skin rash
  • fever
  • sore throat
  • swollen glands
  • unusual bleeding or bruising
  • stomach pains or tenderness
  • Blurred vision or loss of vision, with or without eye pain
  • Convulsions (seizures)
  • Joint pain
  • Mental depression
  • Mood or other mental changes

Common side effects:

  • tingling of hands and feet
  • nausea/vomiting/diarrhea

This is not a complete list of isoniazid side effects. Ask your doctor or pharmacist for more information.

Isoniazid Interactions

Tell your doctor about the medicines you take including prescription and nonprescription medications, vitamins, and herbal supplements. Especially  tell your doctor if you take:

  • acetaminophen (Tylenol)
  • antacids
  • carbamazepine (Tegretol)
  • disulfiram (Antabuse)
  • ketoconazole (Nizoral)
  • phenytoin (Dilantin)
  • theophylline (Theobid, Theo-Dur)
  • valproic acid (Depakene, Depakote)
  • SSRI antidepressants (e.g. fluoxetine [Prozac])

This is not a complete list of isoniazid drug interactions. Ask your doctor or pharmacist for more information.

Isoniazid Precautions

Tell your doctor if you are allergic to isoniazid or any other medicines.

Isoniazid has caused severe and sometimes fatal hepatitis (see "FDA Warning"). Daily ingestion of alcohol may be associated with a higher incidence of isoniazid hepatitis. You should avoid alcohol while taking this medicine.

Tell your doctor if you have any other medical problems such as liver disease, kidney disease, or seizure disorder. With the presence of these conditions, there may be an increase risk of side effects or seizures.

Isoniazid Food Interactions

Medicines can interact with certain foods. In some cases, this may be harmful and your doctor may advise you to avoid certain foods. In the case of isoniazid there are no specific foods that you must exclude from your diet when receiving isoniazid.

Rarely, patients taking isoniazid who eat some foods, such as mature cheese and some fish (e.g. tuna, mackerel, salmon), may experience an exaggerated histamine poisoning reaction e.g. chills, diarrhea, flushing and itching of the skin, headache, lightheadedness, and vomiting.

Inform MD

Before taking isoniazid tell your doctor:

  • if you are allergic to isoniazid or any other medicines.
  • 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 breastfeeding.

Tell your doctor about the medicines you take including prescription and nonprescription medications, vitamins, and herbal supplements. 

Isoniazid and Pregnancy

Tell your doctor if you are pregnant or plan to become pregnant. It is not known if isoniazid will harm your unborn baby.

Isoniazid and Lactation

Tell your doctor if you are breastfeeding or plan to breastfeed. Isoniazid is excreted in human breast milk.

Isoniazid Usage

Isoniazid comes as a tablet, capsule (when combined with rifampin), 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.

If you miss a dose, take it as soon as possible. If it is almost time for the next dose, skip the missed dose.

If you are using the liquid form of isoniazid, use a measuring spoon or device, not a household spoon. This will ensure you get the correct dose.

Isoniazid Dosage

Take isoniazid exactly as prescribed by your doctor. Follow the directions on your prescription label carefully. Dosage is based on your age, weight, medical condition, and response to treatment.

Isoniazid Overdose

If you take too much isoniazid, call your local Poison Control Center or seek emergency medical attention right away.

Other Requirements

  • Store this medicine at room temperature away from heat and moisture.
  • Keep this and all medicines out of the reach of children.

Isoniazid FDA Warning


Severe and sometimes fatal hepatitis associated with isoniazid therapy has been reported and may occur or may develop even after many months of treatment. The risk of developing hepatitis is age related. Approximate case rates by age are: less than 1 per 1,000 for persons under 20 years of age, 3 per 1,000 for persons in the 20-34 year age group, 12 per 1,000 for persons in the 35-49 year age group, 23 per 1,000 for persons in the 50-64 year age group, and 8 per 1,000 for persons over 65 years of age. The risk of hepatitis is increased with daily consumption of alcohol. Precise data to provide a fatality rate for isoniazid-related hepatitis is not available; however, in a U.S. Public Health Service Surveillance Study of 13,838 persons taking isoniazid, there were 8 deaths among 174 cases of hepatitis.

Therefore, patients given isoniazid should be carefully monitored and interviewed at monthly intervals. For persons 35 and older, in addition to monthly symptom reviews, hepatic enzymes (specifically, AST and ALT (formerly SGOT and SGPT, respectively)) should be measured prior to starting isoniazid therapy and periodically throughout treatment. Isoniazid-associated hepatitis usually occurs during the first three months of treatment. Usually, enzyme levels return to normal despite continuance of drug, but in some cases progressive liver dysfunction occurs. Other factors associated with an increased risk of hepatitis include daily use of alcohol, chronic liver disease and injection drug use. A recent report suggests an increased risk of fatal hepatitis associated with isoniazid among women, particularly black and Hispanic women. The risk may also be increased during the post-partum period. More careful monitoring should be considered in these groups, possibly including more frequent laboratory monitoring. If abnormalities of liver function exceed three to five times the upper limit of normal, discontinuation of isoniazid should be strongly considered. Liver function tests are not a substitute for a clinical evaluation at monthly intervals or for the prompt assessment of signs or symptoms of adverse reactions occurring between regularly scheduled evaluations. Patients should be instructed to immediately report signs or symptoms consistent with liver damage or other adverse effects. These include any of the following: unexplained anorexia, nausea, vomiting, dark urine, icterus, rash, persistent paresthesias of the hands and feet, persistant fatigue, weakness or fever of greater than 3 days duration and/or abdominal tenderness, especially right upper quadrant discomfort. If these symptoms appear or if signs suggestive of hepatic damage are detected, isoniazid should be discontinued promptly, since continued use of the drug in these cases has been reported to cause a more severe form of liver damage. Patients with tuberculosis who have hepatitis attributed to isoniazid should be given appropriate treatment with alternative drugs. If isoniazid must be reinstituted, it should be reinstituted only after symptoms and laboratory abnormalities have cleared. The drug should be restarted in very small and gradually increasing doses and should be withdrawn immediately if there is any indication of recurrent liver involvement.

Preventive treatment should be deferred in persons with acute hepatic diseases.