Used for the prevention of cytomegalovirus (CMV) infection and disease in adult recipients of allogeneic hematopoietic stem cell transplants (HSCT), a population at high risk for CMV reactivation. It is particularly beneficial for patients who are CMV-seropositive or who receive stem cells from a CMV-seropositive donor. Letermovir works by inhibiting the CMV terminase complex, which is essential for viral DNA packaging and replication, offering a targeted mechanism distinct from traditional antivirals like ganciclovir. Its favorable safety profile and low myelotoxicity make it suitable for use in the post-transplant period, where bone marrow recovery is critical. Letermovir has been shown in clinical trials to significantly reduce CMV viremia and the need for preemptive antiviral therapies, thereby potentially reducing transplant-related complications and improving patient outcomes.