Tacroclimus is an immunosuppressive drug widely used for the prevention of rejection in organ transplants. It is marketed in two forms: Prograf administered twice a day, and Advagraf, which allows only one administration per day. Advagraf is often used in liver transplantation. Through this clinical case, we demonstrate the role of therapeutic drug monitoring in dosage optimization during the switch to tacrolimus from the Advagraf form to the Prograf form. This is a 60-year-old patient who underwent liver transplantation in 2014, treated with Advagraf 2.5 mg once in the morning. The residual CO concentration was 7.0 ng/ml. Due to the unavailability of Advagraf, this drug was switched to Prograf at the same dosage, 2.5 mg/d in two doses (1.5 mg in the morning and 1.0 mg in the evening). Co increased from 7.0 to 11.0 ng/ml, AUC was 194 ng.h/ml [120-150]. When substituting Advagraf for Prograf, C0 monitoring should be regular and close, and dosage adjustments should be made to ensure that a similar systemic exposure is maintained.