BackgroundThe medication regimen complexity index (MRCI) quantifies patient-level regimen complexity, and higher scores are associated with adverse clinical outcomes. Characterization of regimens using the MRCI for allogeneic hematopoietic cell transplant (allo-HCT) recipients remains unexplored. Regimens may include letermovir which is used for cytomegalovirus prophylaxis and may prevent the need for addition of complex preemptive therapies. However, quantification of complexity in patients receiving letermovir has not been described.ObjectiveThis study aimed to compare MRCI scores over a one-year period in allo-HCT recipients who received letermovir prophylaxis versus those who did not.MethodsA retrospective analysis included adults who underwent allo-HCT from January 1, 2016 to October 31, 2021. MRCI scores were calculated at admission, discharge, day +100, 6 months, and 1-year post-transplant.ResultsA total of 218 patients were included, with 67 receiving letermovir and 151 not receiving letermovir. Median MRCI scores were comparable at discharge post allo-HCT (23 [10-39] vs 22 [12-37], p = 0.97). However, at day +100, patients in the letermovir group exhibited significantly higher median scores compared to the non-letermovir group (59 [46-74] vs 50 [37-67], p = 0.009). By 1-year post allo-HCT, no significant difference in scores was observed between groups (47 [30-68] vs 41 [27-61], p = 0.12).Conclusion and RelevanceThis study revealed increased MRCI scores up to one year after transplantation in allo-HCT recipients receiving letermovir. The nonrandomized study design and potential patient differences between groups complicate the interpretation of the findings. Future analyses should aim to account for these differences.