Used primarily as an anticancer agent, especially in the treatment of solid tumors such as colorectal, breast, and skin cancers. It functions by inhibiting thymidylate synthase, disrupting DNA synthesis and cell division in rapidly growing cancer cells. Commonly administered intravenously, it is also available in topical formulations for treating actinic keratosis and certain superficial skin cancers. Its role in combination therapies enhances the effectiveness of radiation and other chemotherapeutic drugs.