5-Fluorouracil (5-FU) is a chemotherapy medication primarily used to treat various types of cancer, including breast cancer, colon cancer, and stomach cancer. It works by interfering with the synthesis of DNA and RNA, which are essential for cancer cell growth and division. Administered intravenously or topically, 5-FU can be part of combination therapy with other cancer treatments. Common side effects include nausea, vomiting, and a decrease in blood cell counts, which can increase the risk of infections. Regular monitoring is essential to manage these effects effectively.