Irinotecan is a chemotherapy medication primarily used to treat certain types of cancer, including colorectal cancer. It works by inhibiting the enzyme topoisomerase I, which is essential for DNA replication. By disrupting this process, irinotecan helps to prevent cancer cells from dividing and growing. This drug is often administered intravenously and may be used alone or in combination with other treatments. Common side effects include nausea, diarrhea, and fatigue. Patients receiving irinotecan are closely monitored to manage these effects and ensure the treatment's effectiveness.