Do NOT use Isoniazid if:

• you are allergic to any ingredient in Isoniazid or have had severe side effects from isoniazid, such as drug fever, chills, or arthritis
• you have severe liver damage, active liver disease, or liver damage from previous use of Isoniazid
• you have a history of hepatitis caused by any medicine.

Contact your doctor or health care provider right away if any of these apply to you.

Some medical conditions may interact with Isoniazid. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:

• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have diabetes, kidney problems, nerve problems (eg, neuropathy) or risk of nerve problems, HIV, or a history of liver problems
• if you have a history of alcohol or other substance abuse, have unsanitary injectable drug habits, or drink alcohol daily
• if you are older than 35 years old, you have recently given birth, or you have previously taken Isoniazid.

Some medicines may interact with Isoniazid. Tell your health care provider if you are taking any other medicines, especially any of the following:

• Acetaminophen, anticoagulants (eg, warfarin), carbamazepine, hydantoins (eg, phenytoin), rifampin, theophylline, or valproic acid because the risk of their side effects may be increased by Isoniazid
• Ketoconazole because its effectiveness may be decreased by Isoniazid.

This may not be a complete list of all interactions that may occur. Ask your health care provider if Isoniazid may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

Important safety information:

• Check with your doctor before drinking alcohol while using Isoniazid. Alcohol may increase the risk of liver problems. If you have a history of alcohol abuse, you may also be at increased risk of developing nerve problems from Isoniazid. Notify your doctor if you notice any unusual tingling or numbness in your hands or feet.
• If you have a history of diabetes, alcohol abuse, or poor nutrition, your doctor may recommend that you also take vitamin B₆ while you are taking Isoniazid. This may help to decrease your risk of nerve problems. Discuss any questions with your doctor.
• Do not eat foods high in tyramine while you use Isoniazid. Eating foods high in tyramine (eg, aged cheeses, red wines, beer, certain meats and sausages, liver, sour cream, soy sauce, raisins, bananas, avocados) while you use Isoniazid may cause severe high blood pressure. Seek medical attention at once if symptoms of severe high blood pressure occur. These may include severe headache, fast or irregular heartbeat, sore or stiff neck, nausea, vomiting, sweating, enlarged pupils, or sensitivity to light.
• Do not eat foods high in histamine while you use Isoniazid. Eating foods high in histamine (eg, skipjack, tuna, tropical fish) while you use Isoniazid may cause low blood pressure, irregular heartbeat, headache, sweating, or flushing. Contact your doctor at once if any of these symptoms occur.
• Ask your health care provider for a complete list of all foods you should avoid while you are using Isoniazid.
• Isoniazid only works against TB bacteria; it does not treat viral infections (eg, the common cold).
• Be sure to use Isoniazid for the full course of treatment. If you do not, the medicine may not clear up your infection completely. The bacteria could also become less sensitive to this or other medicines. This could make the infection harder to treat in the future.
• Diabetes patients - Isoniazid may affect your blood sugar. Check blood sugar levels closely. Ask your doctor before you change the dose of your diabetes medicine. You may also be at increased risk of developing nerve problems from Isoniazid. Contact your doctor if you notice any unusual tingling in your hands or feet.
• Lab tests, including liver function and eye exams, may be performed while you use Isoniazid. These tests may be used to monitor your condition or check for side effects. Be sure to keep all doctor and lab appointments.
• Use Isoniazid with caution in black and Hispanic women; they may have a greater risk of severe liver problems from Isoniazid.
• Use Isoniazid with caution in patients older 35 years; they may have a greater risk of severe liver problems from Isoniazid.
• Pregnancy and breast-feeding: If you become pregnant, contact your doctor. You will need to discuss the benefits and risks of using Isoniazid while you are pregnant. Isoniazid is found in breast milk. If you are or will be breast-feeding while you use Isoniazid, check with your doctor. Discuss any possible risks to your baby.

All medicines may cause side effects, but many people have no, or minor, side effects.

Check with your doctor if any of these most common side effects persist or become bothersome:

Mild stomach upset.

Seek medical attention right away if any of these severe side effects occur:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); changes in vision; chills or fever; dark urine; general feeling of discomfort; increased thirst or urination; joint pain or swelling; loss of appetite; memory problems; mental or mood changes; nausea; seizures; stomach pain or tenderness; symptoms of low vitamin B₆ levels (eg, confusion, cracks in the corners of the mouth, irritability, mouth redness or soreness, scaly rash); tingling or numbness in the hands or feet; unusual bruising or bleeding; unusual tiredness or weakness; vomiting; yellowing of the skin or eyes.
 This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider.