isoniazid, Nydrazid (discontinued), Laniazid (discontinued), INH (discontinued)

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BRAND NAME: Nydrazid (discontinued), Laniazid (discontinued), INH (discontinued)

DRUG CLASS AND MECHANISM: Isoniazid is an anti-bacterial drug that has been used to prevent and to treat tuberculosis since 1952. Tuberculosis is an infectious disease caused by a bacterium. Once the infection is acquired, it usually remains dormant in the lungs for up to many years. Later, the infection may become active in the lungs and sometimes spreads throughout the body. Patients with a tuberculosis skin test that has recently become abnormal (demonstrating recent infection with tuberculosis) but a normal chest X-ray (demonstrating inactive infection) are given Isoniazid alone for 9 months. Patients with active infection on chest X-ray are given Isoniazid combined with other antituberculous drugs. The mechanism of action of Isoniazid is not known, but it is thought to work through its effects on lipids (fats) and DNA within the tuberculosis bacterium. It is very selective for the tuberculosis bacteria, that is, it has few if any effects on other bacteria.



PREPARATIONS: Tablets: 100 or 300 mg. Syrup: 50 mg per teaspoonful. Injection: 100 mg/ml.

STORAGE: Tablets and syrup should be stored at room temperature, 15-30 C (59-86 F).

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.

PREGNANCY: Isoniazid has not been shown to cause birth defects in humans or animals; however, studies in rats and rabbits have shown that it may increase the risk of fetal death. Nevertheless, tuberculosis is a very serious infection, and many women have been treated with isoniazid during pregnancy with no problems in their infants. Ultimately, the physician and the pregnant patient must evaluate the risks and benefits of isoniazid.

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