• Basic Info
• Top Questions
• Interactions

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).

• disulfiram (Antabuse). Unusual behavior and coordination difficulties may occur if these medicines are taken together.

Isoniazid may increase the effects of the following medicines:

• warfarin (Coumadin);

• carbamazepine (Tegretol);

• cycloserine (Seromycin);

• phenytoin (Dilantin), ethotoin (Peganone), and mephenytoin (Mesantoin);

• meperidine (Demerol);

• benzodiazepines such as alprazolam (Xanax), diazepam (Valium), lorazepam (Ativan), and temazepam (Restoril);

• theophylline (Theo-Dur, Theochron, Theolair, Elixophyllin, Slo-Phyllin, others);

• ethionamide (Trecator-SC).

You may require a dosage adjustment or special monitoring if you are taking any of the medicines listed above.

Drugs other than those listed here may also interact with isoniazid. Talk to your doctor and pharmacist before taking any prescription or over-the-counter medicines, including vitamins, minerals, and herbal products.

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.