Used primarily as an antiviral agent, especially against herpes simplex virus (HSV). It functions by interfering with viral DNA replication, acting as a thymidine analog that gets incorporated into viral DNA, leading to faulty replication and inhibition of viral growth. It has also been used in research settings to study DNA synthesis and cell proliferation. In some cases, it has been explored for oncolytic virus therapies, where it enhances the selectivity and effectiveness of viruses engineered to target cancer cells. Its use is limited clinically due to toxicity concerns, but it remains valuable in laboratory research and virology studies.