09/18/2026
Diarrhea During Cancer Treatment? Don’t Automatically Blame the Treatment
If you’re receiving cancer treatment and develop diarrhea, don’t automatically assume chemotherapy or immunotherapy is the cause.
A recent large U.S. outbreak of Cyclospora, a microscopic intestinal parasite linked to contaminated iceberg lettuce, affected thousands of people. Although that specific outbreak has now ended, Cyclospora and other infections remain important causes of diarrhea.
Symptoms can include frequent watery diarrhea, abdominal cramping, bloating, nausea, loss of appetite, weight loss, and significant fatigue. For people with weakened immune systems, illness may be more severe or prolonged.
The key message: the cause matters because the treatments are different. Your healthcare team may need to specifically request stool testing for Cyclospora, and sometimes more than one sample is needed. Cyclospora is treatable, most commonly with trimethoprim-sulfamethoxazole (TMP-SMX/Bactrim) when appropriate.
If you have cancer and develop persistent or worsening diarrhea, fever, severe abdominal pain, weakness, dizziness, dehydration, or difficulty keeping fluids down, contact your oncology team.