A 17-month-old male patient was diagnosed with T-cell acute lymphoblastic leukemia at 10 months of life, when he was noted to have a white blood cell count of 950,000 with peripheral leukemic blasts as well as systemic symptoms. Subsequently he received multiple courses of chemotherapy, and then underwent a matched unrelated allogeneic stem cell transplant at the age of 15 months. A combination of Busulfan, Fludarabine and Alemtuzumab were utilized for myeloablation prior to allogeneic stem cell transplantation from a matched unrelated donor. Subsequently he was initially started on IV tacrolimus (0.033 mg/kg) for GVHD prophylaxis, and achieved therapeutic levels. Approximately one month after transplant in anticipation of being discharged, the patient was switched to an oral brand name formulation of tacrolimus (Prograf®), and was able to maintain trough levels in the prescribed therapeutic window. The patient was discharged approximately one week later with generic tacrolimus suspension dosed at 0.15 mg/kg PO twice daily which was compounded at an outside pharmacy. Subsequently, he was unable to reach therapeutic levels despite multiple escalations in dosage to a maximum dosage of 0.31 mg/kg PO twice daily. Also during this time the patient’s dose of Voriconazole was reduced from 16.26 mg/kg (therapeutic dosage) to 8.46 mg/kg PO daily (expected prophylactic dosage). During this period when his doses were escalated due to inadequate trough levels, multiple investigations were made, and the pharmacist compounding the medication was contacted. According to the outside pharmacy, the pharmacist compounded the medication in a similar fashion to the inpatient pharmacy and the solvents used were the same. The compounding in the inpatient and outside pharmacy followed a straightforward procedure involving mixing the contents of 6 tacrolimus capsules (5 mg each) with 30 mls of Syrup and 30 mls of oral suspending vehicle. Trough levels were drawn at appropriate times, and the family was compliant with the medication. Initially after transplant the patient manifested evidence of skin GVHD with mild skin erythema; topical steroids were initiated two weeks prior to discharge and were continued as an outpatient. The child’s skin GVHD showed marked improvement with topical steroids yet began to flare a few weeks later when he presented to the clinical pharmacology service for inability to reach a therapeutic level for tacrolimus. Tacrolimus is one of the primary agents used to induce immunosuppression and combat GVHD in bone marrow transplant patients; hence the patient’s resurgence of skin GVHD is likely further manifestation of sub-therapeutic tacrolimus levels. At the time of the initial encounter for skin GVHD and subtherapeutic tacrolimus levels the patient was taking the following medications: acetaminophen (15 mg/kg by oral route every 6 hours as needed for pain for 30 doses), diphenhydramine (1 mg/kg by oral route every 6 hours as needed), famotidine (0.53 mg/kg by oral route twice daily), hydrocortisone 0.5% topical ointment (1 application by topical route twice daily), ondansetron (0.15 mg/kg by oral route every 8 hours as needed for nausea/vomiting), sulfamethoxazole-trimethoprim (13.3 mg/kg/2.6 mg/kg) by oral route twice daily on Monday, Tuesday, Wednesday), valacyclovir (29 mg/kg by oral route every 8 hours), voriconazole (oral suspension 10 mg/kg by oral route twice daily), and multivitamins. Patient had an appropriate response to opiates (including codeine) and other medications per the caregivers. Patient did not have any adverse outcomes with surgery and anesthesia. Review of systems at the initial encounter indicated the patient was irritable due to pruritus. The patient had a generalized rash that caused him to wake at night and necessitated the use of diphenhydramine for symptomatic relief. He also had loose stools yet normal number of bowel movements daily, and was tolerating his diet appropriately. He did not have fever, or a change in appetite or activity. Physical exam showed a fine erythematous rash scattered on face and extremities. Excoriations were present on the lower back and extremities as well. The clinical pharmacology service was consulted at this time to evaluate the etiology of the patient’s inability to reach therapeutic trough levels of tacrolimus.