Diarrhea During Cancer Treatment: How Common and What Helps

Diarrhea is a common side effect of many cancer treatments and can also occur due to the cancer itself or from infections during therapy. ^[1] Diarrhea may be mild, but it can sometimes be serious and needs timely attention to prevent dehydration and complications. ^[2]

How Common Is It?

• Many chemotherapy regimens can cause diarrhea, with reported rates up to 50–80% overall, and severe cases (grade 3–5) in a significant subset, especially with irinotecan- or fluoropyrimidine-based regimens. [PM13] Targeted therapies and immunotherapies can also cause diarrhea, often in up to 60% of users, though severe diarrhea is less frequent than with certain chemotherapies. [PM13]
• Radiation to the abdomen or pelvis, immunotherapy (checkpoint inhibitors), targeted therapies (e.g., EGFR/VEGF pathway inhibitors), and stem cell transplant can all be contributors. ^{[3] [4]}

Why It Happens

• Cancer treatments irritate or inflame the gut lining, alter fluid absorption, or change gut motility, leading to loose stools. [PM13]
• Infections are more common during treatment and can trigger diarrhea; antibiotics used to treat infections can also cause it. ^[3]
• Some cancers themselves (for example, neuroendocrine tumors, colon cancer, lymphoma, medullary thyroid cancer, pancreatic cancer) may cause diarrhea. ^[5]

When to Call Your Care Team

• Contact your doctor urgently if you have signs such as four or more loose stools per day for more than two days, blood in the stool, weight loss from diarrhea, fever ≥38°C (100.5°F), inability to control bowel movements, diarrhea or cramping lasting more than a day, or dizziness with standing. ^[6] Diarrhea that interferes with daily activities also warrants evaluation even if it doesn’t seem severe. ^[6]

First Steps You Can Try at Home

• Hydration: Sip clear liquids (water, oral rehydration solutions, broths, diluted juices) to replace fluids and salts. ^[2]
• Food choices: Many people feel better with small, frequent meals focusing on low‑fiber, bland options such as bananas, rice, applesauce, toast, and plain crackers while avoiding greasy, spicy, very high‑fiber, or very sweet foods. ^[2]
• Track symptoms: Note stool frequency, any blood, cramps, fevers, and what you eat and drink; share this with your care team to guide treatment. ^[2]

Evidence‑Based Medical Management

• Standard antidiarrheals: Loperamide is commonly recommended first line for chemotherapy‑induced diarrhea; dosing is typically higher and more structured than over‑the‑counter labels in oncology protocols, under clinician guidance. [PM14]
• Escalation therapy: If diarrhea persists or is severe, agents such as octreotide may be used; deodorized tincture of opium is another option in selected cases. [PM13]
• Supportive care: Intravenous fluids and electrolyte replacement may be needed for severe dehydration or if you cannot keep fluids down. [PM14]
• Treatment-specific algorithms: Certain drugs (for example, irinotecan or sacituzumab govitecan) have specific management pathways because of characteristic early or late-onset diarrhea patterns. ^{[7] [8]}
• Broader guidance: Clinical resources outline that diarrhea can result from chemotherapy, radiation, immunotherapy, and targeted therapies, and emphasize prompt grading, symptom control, and prevention of complications. ^{[4] [9]}

Special Situations

• Immunotherapy-related diarrhea/colitis: Diarrhea with checkpoint inhibitors can reflect immune-related inflammation and may require steroids or immunosuppressive treatment per oncology guidance. [PM19]
• Targeted therapies: Several oral kinase inhibitors list diarrhea among common side effects; dose adjustments and supportive measures are often used to maintain therapy. [PM22]
• Combination regimens: Some combinations (e.g., irinotecan with fluoropyrimidines) carry higher risk and may need proactive education and rapid escalation plans. [PM13] [PM14]

Practical Tips to Reduce Risk and Discomfort

• Have antidiarrheal medication on hand before cycles start if your regimen is known to cause diarrhea, and know your clinician’s dosing plan. [PM14]
• Keep a hydration plan ready, including oral rehydration solutions, and consider electrolyte beverages if advised. ^[2]
• Ask about diet guidance specific to your treatment, and adjust fiber and lactose based on your tolerance. ^[2]
• Call early if symptoms start; earlier intervention is linked with better control and fewer complications. ^[2] [PM14]

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Quick Reference Table

Topic	Key Points
How common?	Up to 50–80% with some chemo; targeted and immunotherapy also cause diarrhea frequently, though severity varies. [PM13]
Main causes	Chemotherapy, radiation, immunotherapy, targeted therapy, infections, and some cancers themselves. [3] [5] [4]
Red flags	≥4 loose stools/day >2 days, blood, fever ≥38°C, weight loss, incontinence, dizziness, cramps >1 day. [6]
Home measures	Clear liquids, small bland meals, avoid triggers, symptom diary. [2]
Medications	Loperamide first line; consider octreotide or tincture of opium if refractory; IV fluids if needed. [PM13] [PM14]
Special notes	Irinotecan/sacituzumab algorithms; immune-related colitis needs specific management. [7] [8] [PM19]

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Bottom Line

Diarrhea is common with many cancer treatments, but it can usually be managed effectively with early reporting, smart hydration and diet choices, and guideline‑based medicines like loperamide, with escalation (such as octreotide) when needed. [PM13] [PM14] Knowing the warning signs and having a plan with your care team helps prevent dehydration and avoids treatment interruptions. ^{[6] [4]}