DOSING: The initial dose for preventing organ rejection is 3 to 5 mg/kg (oral or IV) daily, starting at the time of transplantation or in some cases 1 to 3 days before transplantation. The initial off-label dose for treating rheumatoid arthritis is 1.0 mg/kg (50 to 100 mg, oral or IV) as a single dose or twice daily. Doses may be increased by 0.5 mg/kg daily up to a maximum dose of 2.5 mg/kg per day. Dosing for other off-label use may vary according to what disease is being treated; in general, off-label use should be done by an experienced medical specialist. Azathioprine should be taken with food.
DRUG INTERACTIONS: Allopurinol (Zyloprim) that is used for treating increased blood levels of uric acid and preventing gout increases azathioprine levels in the blood which may increase the risk of side effects from azathioprine. Therefore, it is important to reduce the dose of azathioprine by approximately 1/3 to 1/4 in patients taking allopurinol. The use of angiotensin-converting enzyme (ACE) inhibitors to control high blood pressure in patients taking azathioprine has been reported to induce anemia (low levels of red blood cells) and severe leukopenia (low levels of white blood cells). Azathioprine reduces blood levels of the blood thinner, warfarin (Coumadin), and thus may reduce the blood thinning effect of warfarin.
PREGNANCY: Azathioprine can cause fetal harm when given to pregnant women and whenever possible should be avoided.
NURSING MOTHERS: Azathioprine is found in breast milk and should not be used in nursing mothers.
SIDE EFFECTS: The most common serious side effects of azathioprine involve the cells of the blood and gastrointestinal system. Azathioprine can cause serious lowering of the white blood cell count, resulting in an increased risk of infections. This effect is reversed when the dose of azathioprine is reduced or temporarily discontinued. Azathioprine can cause nausea, vomiting, and loss of appetite, which may resolve when the daily dose is reduced or divided and taken more than once a day. Azathioprine can cause liver toxicity (for example, in less than 1% of rheumatoid arthritis patients). All patients taking azathioprine require regular testing of blood for blood cell counts and liver tests to monitor for side effects of azathioprine. Other side effects encountered less frequently include fatigue, hair loss, joint pains, and diarrhea.
Reference: FDA Prescribing Information
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