Isoniazid is used with other drugs to treat tuberculosis (TB; a serious infection that affects the lungs and sometimes other parts of the body). Isoniazid is also used with other drugs to treat people with latent (resting or nongrowing) TB including those in close contact with people who have active TB, a positive tuberculin skin test, human immunodeficiency virus (HIV), and those with pulmonary fibrosis (scarring of the lungs with an unknown cause). Isoniazid is in a class of medications called antituberculosis agents. It works by killing the bacteria that cause tuberculosis.

Side Effects Of Isoniazid

Isoniazid may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:

  • upset stomach
  • diarrhea (when taking solution)

If you experience any of the following symptoms or those listed in the WARNING section, call your doctor immediately:

  • eye pain
  • changes in vision
  • numbness or tingling in the hands and feet
  • rash
  • fever
  • swollen glands
  • sore throat
  • unusual bleeding or bruising
  • ongoing pain that begins in the stomach but may spread to the back

Warnings & Precautions

Before taking isoniazid:

  • tell your doctor and pharmacist if you are allergic to isoniazid, any other medications, or any of the ingredients in isoniazid tablets or oral solution. Ask your pharmacist for a list of the ingredients.
  • tell your doctor and pharmacist what other prescription and nonprescription medications, vitamins, nutritional supplements, and herbal products you are taking or plan to take. Be sure to mention any of the following: acetaminophen (Tylenol), antacids, carbamazepine (Carbatrol, Equetro, Tegretol, others), citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac, Sarafem, Symbyax), fluvoxamine (Luvox), ketoconazole (Nizoral), paroxetine (Paxil), phenytoin (Dilantin, Phenytek), sertraline (Zoloft), theophylline (Elixophyllin, Theochron, Theo-24), and valproic acid (Depakene, Depakote). Many other medications may interact with isoniazid, so be sure to tell your doctor about all the medications you are taking, even those that do not appear on this list. Your doctor may need to change the doses of your medications or monitor you carefully for side effects.
  • 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.
  • you should know that you should not drink alcohol while taking this drug.

Isoniazid Dosage

Isoniazid comes as a tablet and a solution (liquid) to take by mouth without food. Isoniazid is usually is taken once a day; it may also be taken one, two, or three times weekly. Take isoniazid at around the same time every scheduled day. 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.

Your doctor may tell you to take isoniazid for 6 months or longer. Continue to take isoniazid even if you feel well. Do not skip doses or stop taking isoniazid without talking to your doctor. Stopping isoniazid too soon may cause bacteria to become resistant to antibiotics.


Before having any laboratory test, tell your doctor and the laboratory personnel that you are taking isoniazid.

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.


All information has been provided courtesy of MedLinePlus from the National Library of Medicine and from the FDA.