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What is isoniazid? Isoniazid is an antibiotic that fights bacteria. Isoniazid is used to treat and to prevent tuberculosis (TB). You may need to take other TB medicines in combination with isoniazid. When treating active TB, isoniazid must be used with other TB medicines. Tuberculosis can become resistant to treatment if isoniazid is used alone. Take all your medicines as prescribed by your doctor. Isoniazid may also be used for purposes not listed in this medication guide. Warnings You should not use isoniazid if you have active liver disease, or if you have taken isoniazid in the past and it caused liver problems, fever, chills, joint pain, or severe allergic reaction. Serious and sometimes fatal liver problems may occur during treatment with isoniazid or after you stop taking this medicine, especially if you are between the ages of 35 and 65. Your liver function may need to be checked every month while you are taking this medicine. Call your doctor right away if you have: nausea, upper stomach pain, loss of appetite, and feeling weak or tired. Avoid drinking alcohol. It may increase your risk of liver damage while you are taking isoniazid. Before taking this medicine You should not use isoniazid if you are allergic to it, or if you have: active liver disease; a history of severe allergic reaction to isoniazid; a history of hepatitis or other liver problems caused by taking isoniazid; or a history of severe isoniazid side effects such as fever, chills, or joint pain and swelling. To make sure isoniazid is safe for you, tell your doctor if you have: a history of liver disease; kidney disease; nerve problems that cause pain, weakness, or numbness; diabetes; HIV or AIDS; if you drink alcohol daily; if you are malnourished; if you use any drugs that are injected; or if you have ever had to stop taking isoniazid for any reason. If you are 35 years or older, your doctor will check your liver enzymes before you start treatment, to make sure you can safely use isoniazid. Serious and sometimes fatal liver problems may occur during treatment with isoniazid or after you stop taking this medication, even months after stopping. The risk of liver problems is highest in adults between the ages of 35 and 65. Serious liver problems may be more likely to occur in women, especially after childbirth, or in women of Hispanic or African-American ancestry. Ask your doctor about your specific risk. It is not known whether this medicine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant. Talk to your doctor if you will be breast-feeding a baby during your treatment with isoniazid. This medicine can pass into breast milk, but it will not treat or prevent tuberculosis in the nursing infant. How should I take isoniazid? Follow all directions on your prescription label. Do not take isoniazid in larger or smaller amounts or for longer than recommended. Take isoniazid on an empty stomach, at least 1 hour before or 2 hours after a meal. Use this medicine for the full prescribed length of time. Your symptoms may improve before the infection is completely cleared. Skipping doses may also increase your risk of further infection that is resistant to antibiotics. Isoniazid will not treat a viral infection such as the flu or a common cold. Your liver function may need to be checked every month while you are taking this medicine. Your doctor may have you take extra vitamin B6 while you are taking isoniazid. Take only the amount of vitamin B6 that your doctor has prescribed. Store at room temperature away from moisture, heat, and light. Keep the bottle tightly closed when not in use. Detailed Isoniazid dosage information What happens if I miss a dose? Take the missed dose as soon as you remember. Skip the missed dose if it is almost time for your next scheduled dose. Do not take extra medicine to make up the missed dose. What happens if I overdose? Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. Overdose symptoms may include vomiting, severe dizziness or drowsiness, slurred speech, blurred vision, hallucinations, trouble breathing, increased thirst, increased urination, fruity breath odor, or loss of consciousness. What should I avoid while taking isoniazid? Avoid drinking alcohol. It may increase your risk of liver damage while you are taking isoniazid. You may need to avoid certain foods while you are taking isoniazid. This includes red wine, aged cheese, dried meats, and tuna or other types of fish. Isoniazid side effects Get emergency medical help if you have signs of an allergic reaction (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning in your eyes, skin pain, red or purple skin rash that spreads and causes blistering and peeling). Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, flu-like symptoms, muscle aches, severe weakness, unusual bruising, or yellowing of your skin or eyes. This reaction may occur several weeks after you began using isoniazid. Call your doctor at once if you have: sudden weakness or ill feeling, or fever for 3 days or longer; pain in your upper stomach (may spread to your back), nausea, loss of appetite; dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes); vision changes, pain behind your eyes; confusion, memory problems, unusual thoughts or behavior; a seizure (convulsions); or pale skin, easy bruising or bleeding (nosebleeds, bleeding gums). Common side effects may include: numbness, tingling, or burning pain in your hands or feet; nausea, vomiting, upset stomach; or abnormal liver function tests. This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088. Isoniazid side effects (more detail) What other drugs will affect isoniazid? Many drugs can interact with isoniazid. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this medication guide. Tell your doctor about all medicines you use, and those you start or stop using during your treatment with isoniazid. Give a list of all your medicines to any healthcare provider who treats you.