Sirolimus is an immunosuppressive drug that inhibits cell proliferation and is given orally to patients who have had an organ transplant. Sirolimus is a compound produced by the bacteria Streptomyces hygroscopicus. It has a half-life of about 60 hours, and is metabolized by the liver into several metabolites that are excreted from the body in the stool (more than 90%) and urine (less than 2%). Like other immunosuppressive agents, sirolimus may cause side effects and adverse reactions and is associated with an increased risk of infection and the development of lymphoma. This test is ordered to measure the amount of the drug in the blood to determine whether concentrations have reached therapeutic levels, and is below toxic levels. Sirolimus may decrease a person’s platelet count, increase their blood lipids, and, when taken long-term with other immunosuppressants, affect kidney function. Side effects may include: anemia, constipation, diarrhea, fever, headache, hypertension, insomnia, nausea, sensitivity to sunlight, and weight gain.