Chemotherapy and Blood in Stool: Causes, Care, and Coping

Chemotherapy can be associated with blood in the stool, and it’s something your care team generally wants to know about promptly. Certain regimens can irritate or damage the lining of the gut (mucositis) or raise bleeding risk, and care instructions commonly advise calling your team if you notice blood in bowel movements. ^[1] Blood appearing very dark or black (tarry) is also a warning sign that needs medical evaluation. ^[2]

Why It Can Happen

• Gut lining irritation (mucositis): Some chemotherapy drugs inflame or injure the lining from mouth to rectum, which can lead to soreness, diarrhea, and occasionally bleeding. ^[3] Ongoing rectal irritation or anal discomfort that does not improve, especially with diarrhea, is a reason to contact your clinician. ^[4]
• Low platelets (thrombocytopenia): Chemotherapy can lower platelets, which help blood clot, increasing the chance of bruising and bleeding, including blood in stool or around the anus. ^[5] When platelets are low, extra caution is advised to reduce bleeding risk. ^[6]
• Diarrhea- or constipation-related injury: Frequent loose stools or straining with hard stools can irritate or tear anal tissue, causing bright red blood on toilet paper or in the bowl. Guidance for chemotherapy side effects specifically flags blood in stool or around the anus as a reason to seek advice. ^{[7] [8]}

When to Seek Medical Help

• Any blood in bowel movements: Report it to your oncology team; this is a standard “call your provider” trigger during chemotherapy. ^[1]
• Very dark/black stools (tarry): This may suggest bleeding higher up in the gut and requires prompt attention. ^[2]
• Bleeding plus other symptoms: New or worsening belly cramps with diarrhea, fever, or ongoing diarrhea despite simple measures warrant a call. ^[7]
• Signs of broader bleeding: Easy bruising, nose or gum bleeding, or red eye patches may signal low platelets and deserve timely review. ^[5]

How to Cope at Home (While You Contact Your Team)

• Be gentle on the rectal area: Avoid enemas and rectal suppositories during chemotherapy unless your team specifically approves, as they can worsen irritation and bleeding. ^[2]
• Hydrate and protect the gut: If you have diarrhea, your team may advise a short period of bland foods and adequate fluids while they evaluate; persistent diarrhea still needs a call, especially if accompanied by bleeding. ^[1]
• Prevent constipation: Stool softeners and fiber can help, but if you haven’t had a bowel movement in more than three days or you notice blood, reach out for guidance rather than escalating home remedies. ^[8]
• Reduce bleeding risks if platelets might be low: Use a soft toothbrush, avoid vigorous straining, and discuss any blood thinners or supplements with your team to minimize bleeding risk. ^[6]

What Your Care Team May Do

• Assess severity and source: They’ll ask about stool color (bright red vs. dark/black), frequency, pain, diarrhea/constipation, and any other bleeding signs. This helps distinguish minor anal irritation from more significant gastrointestinal bleeding. ^{[1] [2]}
• Check labs: A blood count can look for low platelets that increase bleeding risk. ^[5]
• Adjust medications: They may prescribe anti-diarrheals, topical treatments for anal irritation, or modify chemotherapy timing/dose if needed to protect the gut lining. ^[3]
• Review anticoagulants: If you’re on blood thinners, your team may reassess interactions with anti-cancer drugs because combined effects can raise bleeding risk. ^{[9] [10]}

Practical Do’s and Don’ts

• Do:
  • Notify your oncology clinic if you see blood, even if it seems small. ^[1]
  • Watch for black, tarry stools and call promptly if present. ^[2]
  • Keep hydrated and use gentle, non-irritating hygiene for the anal area. ^[1]
• Don’t:
  • Use enemas or rectal suppositories unless your team approves. ^[2]
  • Ignore persistent diarrhea, new cramps, or fever alongside bleeding. ^[7]
  • Assume it’s “normal” your team expects and wants to hear about bleeding symptoms during treatment. ^[1]

Bottom Line

• Blood in stool can occur during chemotherapy due to gut lining irritation, diarrhea/constipation-related injury, or low platelets, and it should be reported. ^{[3] [5] [7]}
• Very dark/black stools, any visible blood, or bleeding with other symptoms are red flags to contact your care team promptly so they can keep you safe and comfortable. ^{[2] [1] [7]}