isoniazid (Nydrazid, Laniazid, INH are all discontinued brands) (cont.)

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PRESCRIBED FOR: Isoniazid is used to prevent active tuberculosis in persons who have an abnormal skin test for tuberculosis (latent tuberculosis) or in combination with other drugs for the treatment of active tuberculosis.

DOSING: The recommended dose for latent tuberculosis 300 mg daily for 9 months. Active tuberculosis is treated with 5 mg/kg up to 300 mg daily or 15 mg/kg up to 900 mg 1-3 times weekly. It is best to take Isoniazid on an empty stomach for maximum absorption into the body, but if it causes abdominal discomfort, it may be taken with food or with a non-aluminum antacid such as Tums or Titralac. (Aluminum-containing antacids bind to the Isoniazid in the intestine and prevent its absorption.)

DRUG INTERACTIONS: Antacids containing aluminum (for example, Mylanta; Maalox, Gelusil; Amphojel; Alternagel) reduce the amount of Isoniazid that is absorbed from the intestine, and this can result in reduced blood levels and effect of Isoniazid. If aluminum-containing antacids must be taken, they should be taken at least one hour after the Isoniazid to prevent binding.

Isoniazid can increase the effectiveness of the blood thinner, warfarin (Coumadin) by interfering with the enzyme in the liver that eliminates warfarin.

Isoniazid can increase the effect of certain benzodiazepines, such as diazepam (Valium), triazolam (Halcion), and others, by interfering with the enzymes in the liver that eliminate benzodiazepines. This may result in excessive sedation.

Carbamazepine (Tegretol) taken at the same time as isoniazid, can result in an increased risk of side effects from both carbamazepine and isoniazid. Isoniazid can decrease the rate at which the liver eliminates phenytoin (Dilantin), which can raise the blood levels and result in side effects of phenytoin. Because rifampin can be toxic to the liver, the use of rifampin and Isoniazid together increases the risk of liver toxicity to a level that is greater than with either drug alone.

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