Irinotecan is a chemotherapy drug commonly used in the treatment of colorectal cancer, particularly in advanced or metastatic settings.

It belongs to a class of drugs known as topoisomerase I inhibitors.

Irinotecan works by blocking topoisomerase I, an enzyme involved in DNA replication. This interference may cause DNA damage in cancer cells, preventing them from dividing and potentially leading to cell death.

Because it also affects normal rapidly dividing cells, side effects such as diarrhea, fatigue, and bone marrow suppression may occur and often require careful management.