Used in the treatment of viral infections, particularly herpes simplex virus (HSV) and varicella-zoster virus (VZV). It acts as a nucleoside analog that inhibits viral DNA replication by interfering with the viral DNA polymerase enzyme. Due to its potent antiviral activity, it has been investigated for use in immunocompromised patients with severe herpesvirus infections. However, its clinical use is limited due to potential drug interactions, especially with 5-fluorouracil, which can lead to severe toxicity.