Is diarrhea a common side effect of skin cancer treatment?

Diarrhea can occur with several skin cancer treatments, especially systemic therapies like chemotherapy, immunotherapy, targeted therapy, and radiation that involves the abdomen or pelvis, and it sometimes relates to infections, antibiotics, or the cancer itself. ^[1] Diarrhea is a recognized side effect during cancer care and may range from mild to severe, so knowing when to self-manage versus call your care team is important. ^{[2] [3]}

Why diarrhea happens with skin cancer treatments

• Chemotherapy, radiation, immunotherapy, targeted therapy, and bone marrow transplant can disrupt the gut lining and cause diarrhea. ^[2]
• Cancer treatments can lower defenses against infections; infections and the antibiotics used to treat them can trigger diarrhea. ^[2]
• The cancer itself can sometimes contribute to diarrhea, though this is less common in skin cancers compared with some other cancers. ^[4]

Special note on immunotherapy-related diarrhea

Immune checkpoint inhibitors (e.g., anti–PD-1, anti–PD-L1, anti–CTLA-4), often used for melanoma and some advanced skin cancers, can cause immune-related diarrhea or colitis that ranges from loose stools to severe inflammation of the colon. [PM13] This type often responds to steroids, and some cases need additional biologic therapy if steroids are not enough. [PM13] Because infections can look similar, stool pathogen testing is commonly recommended when new diarrhea starts on immunotherapy. [PM15]

When to contact your care team urgently

• Six or more loose bowel movements a day for more than two days. ^[3]
• Blood in stool, black/tarry stools, fever, severe cramps, or signs of dehydration (very thirsty, dry mouth/skin, less urine, dark urine, dizziness, or weakness). ^{[3] [5]}
• Any moderate-to-severe diarrhea while on immunotherapy, or diarrhea that wakes you at night or is accompanied by abdominal pain. [PM13]

What you can try at home for mild symptoms

• Hydration: Sip fluids often; choose oral rehydration solutions, broths, or diluted juices to replace water and electrolytes. ^[6]
• Diet: Small, frequent meals; bland, lower-fiber choices (e.g., bananas, rice, applesauce, toast) and avoid greasy, very spicy, or high‑sugar foods if they worsen symptoms. ^[6]
• Skin care: Clean the anal area gently with warm water or nonirritating wipes and pat dry; apply a moisture barrier ointment (e.g., petroleum jelly) to protect skin. ^[7]

Medication safety and options

• Avoid starting over‑the‑counter antidiarrheals without checking with your oncology team if you are on chemotherapy or immunotherapy, because some medicines can be risky in this setting. ^[6]
• If diet changes aren’t enough, your clinician may prescribe medications tailored to the cause and severity, and they will advise on when to pause or modify cancer therapy. ^[7]
• For suspected immune-related colitis, clinicians typically use corticosteroids first; some cases need additional biologics if symptoms don’t improve. [PM13]

Clinical management principles your team may use

• Assess severity (number of stools/day over baseline, presence of blood or pain) and dehydration risk; check for infections when appropriate. ^{[3] [5]}
• Provide stepwise treatment: fluids, diet, antidiarrheals when safe, and escalate to steroids/biologics for immune-related colitis. ^[6] [PM13]
• Adjust cancer treatment as needed until diarrhea is controlled, then consider cautious resumption depending on the cause and response. [PM13]

Red flags for emergency care

• Severe dehydration symptoms, fainting, inability to keep fluids down, bloody stools, or severe abdominal pain should prompt urgent evaluation. ^{[3] [5]}

Staying in close contact with your oncology team and reporting new or worsening diarrhea early helps prevent complications and allows safe, effective treatment adjustments. ^[3]