Can Immunotherapy Cause Blood in Stool, and How Can You Cope?

Yes, immunotherapy can cause blood in the stool in some people, most often from inflammation in the large intestine called immune-related colitis. This side effect is recognized across multiple checkpoint inhibitors (such as PD-1, PD-L1, and CTLA-4 drugs) and is considered a warning sign that needs prompt medical attention. Blood in stool, black/tarry stool, more frequent or watery bowel movements, and abdominal pain or cramping are typical red-flag symptoms of this condition. ^{[1] [2] [3] [4] [5] [6] [7]}

Why It Happens

• Immune checkpoint inhibitors can over-activate the immune system and trigger inflammation in the gut (colitis), which can lead to diarrhea and bleeding. ^{[2] [8]}
• This can appear as bright red blood mixed with stool, blood on toilet paper, or black/tarry stools if bleeding is higher up in the gastrointestinal tract. ^{[1] [3] [4] [5]}

Warning Signs That Need Urgent Care

Seek immediate medical advice or go to urgent care/emergency services if you have any of the following:

• Blood in stool or black, tarry, sticky stool. ^{[1] [3] [4] [5] [6]}
• Severe or worsening abdominal pain, cramping, or tenderness. ^{[1] [3] [4] [5] [6]}
• Diarrhea that is more than 3 watery stools in a day or is rapidly increasing. ^{[9] [10]}
• Dizziness, light-headedness, or signs of dehydration or significant blood loss. ^{[11] [12]}

What To Do Right Away

• Contact your oncology team as soon as you notice blood in the stool; do not wait for it to pass on its own. Early treatment prevents complications. ^{[1] [2]}
• Avoid taking over-the-counter anti-diarrheal medicines (like loperamide) unless your care team specifically tells you to, because treatment differs if colitis is immune-related. ^{[9] [13]}

How Clinicians Usually Manage It

• Your team may pause immunotherapy and evaluate you for immune-related colitis. ^[8]
• If colitis is suspected, treatment often includes corticosteroids to calm inflammation; more severe or steroid-refractory cases may need other immune-modulating medicines as guided by your oncology and GI teams. ^[8]
• For some regimens, patient information sheets specifically highlight bleeding and colitis symptoms and instruct immediate reporting or emergency evaluation when severe. ^{[11] [6] [12]}

Practical Coping Steps at Home (While You Await Guidance)

These tips are supportive and do not replace medical care:

• Stay hydrated with clear fluids and oral rehydration solutions if you have diarrhea. Report any signs of dehydration promptly. ^{[9] [10]}
• Follow a gentle diet (e.g., bananas, rice, applesauce, toast) if your team allows, and avoid high-fiber, spicy, or fatty foods that may worsen diarrhea. ^{[9] [10]}
• Keep a symptom log: number of bowel movements, presence of blood, stool appearance (red vs. black/tarry), abdominal pain level, and associated signs like dizziness. Share this with your care team. ^{[1] [9]}
• Do not start or stop any medications without your oncology team’s approval, including anti-diarrheals and NSAIDs. ^{[9] [13]}

When Symptoms Typically Start

Mouth and gut side effects can begin within days to weeks after starting immunotherapy, but timing varies bleeding can occur at any point during treatment or even after it ends, so continued vigilance is important. ^{[14] [2]}

Summary

• Blood in the stool can occur with immunotherapy and often signals immune-related colitis that needs prompt evaluation. Early, appropriate treatment is important to prevent complications and is usually very effective. ^{[1] [2] [3] [4] [5] [6] [7] [8]}
• Avoid self-treating diarrhea without guidance, and call your oncology team if you notice bleeding or a significant change in bowel habits. ^{[9] [13]}

Quick Reference Table: Red Flags and Actions

Situation	What it may mean	What to do
Bright red blood in stool or on paper	Lower GI bleeding from colitis	Call your oncology team immediately; seek urgent care if heavy bleeding or severe pain. [1] [2]
Black, tarry, sticky stool	Possible upper GI bleeding	Seek urgent evaluation immediately. [1] [3] [4] [5]
>3 watery stools/day or rapidly increasing frequency	Possible immune-related colitis	Call your care team the same day; don’t use anti-diarrheals without approval. [9] [10] [13]
Severe abdominal pain, cramping, tenderness	Moderate–severe colitis	Urgent medical assessment is needed. [1] [3] [4] [5] [6]
Dizziness, light-headedness, pale or weak	Possible significant fluid loss or bleeding	Go to emergency services. [11] [12]

If you experience any of the above symptoms during immunotherapy, contacting your oncology team quickly and following their instructions is the safest path. ^{[1] [2] [9] [13]}