A 24-year-old woman with advanced Hodgkin disease received the standard dosing protocol for busulfan/cyclophosphamide before allogeneic hematopoietic stem cell transplantation (HSCT)1 from a matched unrelated donor. The target area under the plasma concentration vs time curve (AUC) for busulfan was set at 950 μmol · L−1 · min−1, near the low end of the therapeutic interval of 900–1350 μmol · L−1 · min−1. The patient’s body mass index (BMI) was 45.5 kg/m2 (height, 170.2 cm; weight, 132.0 kg), so the dose was based on the patient’s ideal body weight. The patient received 2-hour intravenous infusions of busulfan every 6 h for 4 days (16 doses total). The patient was concurrently prescribed multiple medications, including an immunosuppressant, an antiviral, an antifungal, an antidepressant, an anxiolytic, a β-blocker, and a muscle relaxant, as well as antibiotics, warfarin, opioids, and antiepileptic drugs.