You will be closely monitored (for example, by having blood tests) by your doctor while using this medication. Keep all medical and lab appointments.

Irinotecan may cause severe diarrhea, which can occur during or right after you receive this medication and/or more than 24 hours afterward. If the diarrhea starts right away, you may also have other side effects such as runny nose, increased saliva, watery eyes, sweating, stomach cramps, or flushing. If the diarrhea starts later, it could be a different type that may not stop and can cause serious (possibly fatal) loss of too much body water (dehydration), mineral imbalance, or serious infection (sepsis). Tell your doctor right away if you have diarrhea and/or any of the symptoms that occur with the early form of diarrhea. Also tell your doctor if you have stomach/abdominal pain, extreme thirst, very dry mouth, muscle cramps/weakness, fast/slow/irregular heartbeat, fainting, dizziness, lightheadedness, or signs of infection (such as sore throat that doesn't go away, fever). Your doctor should prescribe other medications to treat the diarrhea and other symptoms.

This medication may cause very serious (possibly fatal) blood disorders (decreased bone marrow function leading to low number of blood cells such as white cells, red cells, and platelets). This effect can decrease your body's ability to fight an infection, cause your body to bruise or bleed easier, or cause anemia. Tell your doctor right away if you develop any of the following: signs of infection (such as sore throat that doesn't go away, fever), easy bruising/bleeding, unusual tiredness, fast/pounding heartbeat.