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Tacrolimus (FK506, Prograf, etc.) is a drug that decreases immune system response. It is used alone or in combination with other drugs to prevent transplant rejection in transplant programs. Tacrolimus is metabolized by the liver, and several drugs can increase (anticonvulsants, rifampin) or decrease (calcium channel blockers, antifungal agents, some antibiotics, grapefruit juice) effective blood levels of tacrolimus. Desirable tacrolimus levels vary depending on the type of transplant, risk of rejection, length of time since transplantation, use of other immunosuppressive drugs and side effects. The lowest blood levels between two doses are measured, usually 12 hours after the last dose, or immediately before the next dose of the drug. Results are expressed in micrograms of tacrolimus per litre of whole blood (µg/L).

In the long term, optimal blood levels of tacrolimus are generally kept between 5.0 and 15.0 µg/L. However, higher levels are desired immediately after transplantation and are gradually reduced in stable patients. In the longer term, levels below 20 µg/L are desirable to avoid significant side effects of the drug.

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