Irinotecan is used alone or in combination with other medications to treat colon or rectal cancer (cancer that begins in the large intestine). Irinotecan is in a class of antineoplastic medications called topoisomerase I inhibitors. It works by stopping the growth of cancer cells.Irinotecan comes as a liquid to be given over 90 minutes intravenously (into a vein) by a doctor or nurse. It is usually given not more often than once a week, according to a schedule that alternates one or more weeks when you receive irinotecan with one or more weeks when you do not receive the medication. Your doctor will choose the schedule that will work best for you.