Letermovir is primarily used in the prevention of cytomegalovirus (CMV) infection in adult patients who have undergone an allogeneic hematopoietic stem cell transplant. It works by inhibiting the viral terminase complex, which is essential for the replication of CMV. This medication is particularly valuable in transplant recipients, as they are at high risk for CMV reactivation due to their compromised immune systems. By preventing CMV infection, letermovir helps reduce the risk of complications such as CMV disease, which can affect multiple organs and lead to severe outcomes. Its use is typically initiated after the transplant and continued for a specified period to ensure effective prophylaxis.