• Basic Info
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What is isoniazid?
Isoniazid is an antibiotic. It prevents tuberculous bacteria from multiplying in the body. Isoniazid is used to treat and to prevent tuberculosis (TB). Isoniazid may also be used for purposes other than those listed in this medication guide.

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What are the possible side effects of isoniazid?

If you experience any of the following serious side effects, stop taking isoniazid and seek emergency medical attention or contact your doctor immediately:

• an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives);

• unusual weakness or fatigue;

• nausea, vomiting, or loss of appetite;

• abdominal pain;

• yellow skin or eyes;

• dark urine;

• numbness or tingling in your hands or feet;

• seizures;

• blurred vision; or

• confusion or abnormal behavior.

Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.

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How should I take isoniazid?
Take isoniazid exactly as directed by your doctor. If you do not understand these instructions, ask your pharmacist, nurse, or doctor to explain them to you. Take each dose with a full glass (8 ounces) of water. Take isoniazid on an empty stomach 1 hour before or 2 hours after meals. If nausea occurs, ask your doctor if you can take isoniazid with food. Take all of the isoniazid that has been prescribed for you even if you begin to feel better. Your symptoms may begin to improve before the infection is completely treated.

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What is the most important information I should know about isoniazid?
Take all of the isoniazid that has been prescribed for you even if you begin to feel better. Your symptoms may begin to improve before the infection is completely treated. Avoid alcohol while taking isoniazid. Alcohol may increase the risk of damage to the liver during isoniazid treatment. Contact your doctor immediately if you experience numbness or tingling in the hands or feet, weakness, fatigue, loss of appetite, nausea and vomiting, yellowing of the skin or eyes, or darkening of the urine.

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What happens if I miss a dose?
Take the missed dose as soon as you remember. If it is almost time for the next regularly scheduled dose, skip the missed dose and take the next one as directed. Do not take a double dose of this medication.

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What other drugs will affect isoniazid?
Before taking isoniazid, tell your doctor if you are taking any of the following drugs: antacids. Aluminum salts will decrease the action of isoniazid. Do not take antacids that contain aluminum for 1 to 2 full hours before or after taking a dose of isoniazid. ketoconazole (Nizoral) and itraconazole (Sporanox). Isoniazid may decrease the actions of these antifungal medicines. Therefore, a fungal infection may not be adequately treated. rifampin (Rimactane, Rifadin) or rifabutin (Mycobutin).

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Can I take this if I am pregnant or trying to get pregnant or if I am breastfeeding?

Isoniazid has been assigned to a pregnancy category C by the FDA. There are no controlled data in human pregnancies. Embryocidal effects were noted in both rats and rabbits after administration of isoniazid orally during pregnancy. While cases of suspected isoniazid induced anomalies have been reported, causality is unknown and retrospective analyses have failed to document significant teratogenic risk. Isoniazid should only be given during pregnancy when need has been clearly established. The initial therapy of active tuberculosis in pregnant patients recommended by the Centers for Disease Control includes isoniazid. Isoniazid is also recommended by the American Thoracic Society. For HIV-infected women, the USPHS/IDSA (U.S. Public Health Service/Infectious Diseases Society of America) Prevention of Opportunistic Infections Working Group recommends that chemoprophylaxis be given during pregnancy to those with a positive tuberculin skin test or a history of exposure to active tuberculosis, after active disease has been ruled out. Isoniazid is the prophylactic agent of choice unless exposure to drug-resistant tuberculosis is suspected. However, the Working Group also states that "because of theoretical concerns regarding possible teratogenicity associated with drug exposures during the first trimester, providers may choose to initiate prophylaxis after the first trimester."

Isoniazid crosses the placenta. At delivery, serum cord concentrations have been reported to range from 0% to 66% of that found in maternal serum. Reports of the use of isoniazid during pregnancy generally involve multiple drug therapy. Malformations reported which have involved isoniazid use include one case each of myelomeningocele and hypospadias. The Collaborative Perinatal Project reported 85 fetal exposures to isoniazid during the first trimester. Ten malformations were reported, which was nearly twice the normal rate. However, due to the small number of exposures, no conclusion could be drawn. Hemorrhagic complications resulting from vitamin K deficiency have been reported in two infants whose mothers received antituberculous drugs. One mother had been treated with isoniazid and rifampin, and the other with isoniazid, rifampin, and ethambutol. A third infant with hemorrhagic disease had only been exposed to ethambutol and rifampin. Pleural mesothelioma was reported in a nine-year-old child whose mother received isoniazid during the second and third trimesters of pregnancy. The carcinogenicity of isoniazid has not been documented.

Isoniazid and acetylisoniazid, a metabolite, are excreted into human milk. Following oral administration of 300 mg, approximately 7 mg was excreted in breast milk over 24 hours. While adverse effects in the nursing infant are not reported, it may be prudent to monitor the infant for signs of peripheral neuritis and hepatotoxicity. Isoniazid is considered compatible with breast-feeding by the American Academy of Pediatrics.

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What does my medication look like?
Isoniazid is available with a prescription in tablet, syrup, and injection formulations. Ask your pharmacist any questions you have about this medication, especially if it is new to you.

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What happens if I overdose?
Seek emergency medical attention if an overdose is suspected. Symptoms of an isoniazid overdose include nausea, vomiting, dizziness, slurring of speech, blurred vision, visual hallucinations, seizures, coma, and death.

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What should I discuss with my healthcare provider before taking isoniazid?
Before taking this medication, tell your doctor if you have ever had an allergic reaction to isoniazid, kidney disease, or liver disease. You may not be able to take isoniazid, or you may require a dosage adjustment or special monitoring during treatment if you have any of the conditions listed above. It is not known whether isoniazid will be harmful to an unborn baby. Do not take this medication without first talking to your doctor if you are pregnant or could become pregnant during treatment.

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Can I stop taking the medication if I feel better?
If you have been diagnosed with a disease for which an antibiotic is needed, you must complete the prescribed course of treatment. Even if you start to feel better, do not skip any doses and remember to take the medication until it is all gone.

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I am on so many medications; do I have to take them all?
This is called polypharmacy—many different medications being used at the same time by one person. Sometimes, being on multiple medications is acceptable and appropriate but at other times it may be problematic. If you are receiving your medications from multiple physicians you need to ensure that they all know what medications you are taking. The best way to do this is to make a list of all the medications you are currently using, including all nutritional supplements, homeopathic remedies, vitamins and over-the-counter drugs (if possible, also include all the diseases you have been diagnosed with). Give a copy to every doctor who takes care of you so they have it on file, this way they can avoid duplicating medications and perhaps even try to consolidate some. After every doctor's visit remember to update the list accordingly. Also, as much as you possibly can, try to use the same pharmacy to fill all your prescriptions, this way any potential drug interactions can be caught and averted.

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