Can Melanoma Cause Blood in the Stool? Warning Signs, Urgency, and Tests for Gastrointestinal Involvement

Melanoma can involve the gastrointestinal (GI) tract and, in some cases, may lead to blood in the stool (visible bleeding or occult blood detected on tests). ^[1] GI involvement most often affects the small bowel and can present with nonspecific symptoms such as abdominal pain, weight loss, and sometimes GI bleeding, which means blood in stools may be an early or urgent clue in someone with a current or past melanoma diagnosis. ^[2] While GI metastases are not rare at autopsy, they often remain silent; when symptoms appear, evaluation should not be delayed due to the potential for obstruction, intussusception, or significant bleeding. ^{[3] [2]}

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How Melanoma Reaches the GI Tract

• Melanoma commonly spreads to lymph nodes, liver, lungs, bone, and brain; the GI tract especially the small intestine is also a known site, even though it is discussed less often. ^[4]
• Melanoma inside mucous membranes (mucosal melanoma) can arise in the GI tract or anus without a skin primary; these forms are rare but bleeding and pain in the anal area can be presenting signs. ^{[5] [6]}

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Red-Flag Symptoms Requiring Prompt Attention

If you have a history of melanoma (even early-stage skin melanoma) and develop new GI symptoms, clinicians generally recommend a low threshold to investigate, because symptoms can be vague yet important. ^{[1] [2]}

• Dark, tarry stools (melena) or visible blood in the stool. This may reflect bleeding in the upper or lower GI tract and needs urgent assessment to rule out metastasis and other causes. ^[2]
• New or persistent abdominal pain, bloating, or cramps, especially with nausea or vomiting. These may suggest obstruction or intussusception from a small bowel lesion. ^{[3] [2]}
• Unintentional weight loss or iron-deficiency anemia, which can be due to chronic occult bleeding. ^{[1] [2]}
• Changes in bowel habits (diarrhea or constipation), particularly if persistent. ^[3]
• Anal pain or bleeding, which can be a sign in anorectal mucosal melanoma. ^{[5] [6]}

For people undergoing immunotherapy (e.g., ipilimumab and nivolumab), severe diarrhea, uncontrolled bleeding, dizziness/light-headedness, or dark, tarry stools warrant immediate contact with your care team or an emergency department, because immune‑related colitis and bleeding can become serious quickly. ^{[7] [8]}

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When Is It Urgent?

• Any new GI bleeding (black stools, maroon stools, bright red blood) in someone with a current or past melanoma should be evaluated urgently to prevent complications and to detect treatable lesions. ^[2]
• Symptoms of obstruction (severe abdominal pain, vomiting, inability to pass gas or stool) or signs of intussusception/perforation are emergencies and require immediate medical attention. ^[3]
• Rapidly worsening fatigue, pallor, or shortness of breath can indicate significant blood loss or anemia from ongoing bleeding and should be assessed promptly. ^[2]

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Recommended Tests to Evaluate GI Involvement

Because GI melanoma can be hard to detect and symptoms are nonspecific, doctors often combine imaging and endoscopy to locate and confirm lesions. ^{[3] [1]}

• CT scan of the abdomen/pelvis: Common first-line imaging to look for masses, obstruction, or bleeding sources; widely used in case series of GI melanoma metastases. ^[1]
• PET‑CT: Helpful to identify metabolically active metastases and guide where to look further; may raise suspicion for small-bowel lesions not visible on routine studies. ^[9]
• Capsule endoscopy or device-assisted enteroscopy: Especially useful for small bowel, which is beyond the reach of standard scopes; these can reveal bleeding sites and tumors. ^[9]
• Upper endoscopy (EGD) and colonoscopy: Evaluate for sources in the esophagus, stomach, duodenum, colon, and rectum; critical when bleeding pattern suggests upper or lower GI origin. ^[2]
• Laboratory tests: Complete blood count (to check for anemia), iron studies (for occult blood loss), and LDH (sometimes linked with disease burden and prognosis). ^{[9] [1]}

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What Happens if GI Metastases Are Found?

• Surgical resection of isolated or limited GI metastases can improve symptoms (e.g., stop bleeding or relieve obstruction) and offers a survival benefit in selected patients, even though overall prognosis varies. ^{[1] [9]}
• Even patients without an obvious skin primary may have GI melanoma; resection and multidisciplinary care are still considered. ^[1]
• After resection, ongoing systemic therapy decisions depend on disease spread, performance status, and molecular/immunologic factors, but complete resection when feasible is associated with better symptom control and, in some cases, prolonged survival. ^{[9] [1]}

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Special Note on Mucosal Melanoma

Mucosal melanoma, although rare, can start in the GI tract or anus and often presents differently than typical skin melanoma. Anal bleeding or pain, diarrhea, or constipation can be signals that warrant focused anorectal evaluation. ^{[5] [6]} Because it is internal and not related to sun exposure, early symptom recognition matters to initiate the right tests. ^[6]

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Practical Takeaways

• Melanoma can involve the GI tract and cause blood in the stool, though symptoms are often subtle at first. ^[2]
• New GI bleeding, persistent abdominal pain, weight loss, or anemia in someone with a melanoma history should prompt urgent medical evaluation. ^{[1] [2]}
• CT and PET‑CT, along with endoscopic methods (including capsule endoscopy), are frequently used to find and confirm GI metastases, especially in the small bowel. ^{[9] [1]}
• In selected cases, surgical removal of GI metastases can control bleeding and obstruction and may extend survival, within a comprehensive cancer-care plan. ^{[1] [9]}
• If you are on immunotherapy, severe diarrhea, uncontrolled bleeding, tarry stools, dizziness, or light‑headedness should trigger immediate contact with your oncology team or an emergency department. ^{[7] [8]}

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Frequently Asked Questions

Is screening for GI spread routine in melanoma?

Routine screening for GI metastases is not typically done in asymptomatic individuals, but clinicians recommend a low threshold to investigate when symptoms appear due to the risk of delay and poor outcomes without timely diagnosis. ^{[2] [1]}

Can early-stage skin melanoma still spread to the GI tract?

Although uncommon, GI metastases have been documented even after early-stage cutaneous melanoma; therefore new GI symptoms should not be dismissed solely based on the original tumor being thin. ^[1]

Are dark, tarry stools the same as blood in the stool?

Dark, tarry stools (melena) generally indicate digested blood from upper GI bleeding, whereas bright red blood often suggests lower GI bleeding; both warrant urgent evaluation in the context of melanoma history. ^[2]

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What to Do Now

• If you notice blood in your stool, black/tarry stools, or new persistent GI symptoms and have a current or past diagnosis of melanoma, seek urgent medical evaluation to arrange appropriate imaging and endoscopic tests. ^{[2] [1]}
• If you are receiving immunotherapy and experience severe diarrhea, bleeding, or light‑headedness, contact your care team or go to the emergency department right away, as immune‑related complications can become serious quickly. ^{[7] [8]}

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Sources

• GI metastases symptoms (abdominal pain, weight loss, GI bleeding), urgency of evaluation and nonspecific presentation; not routinely screened but should be investigated when symptomatic. ^[2]
• Broad spectrum of small-bowel melanoma presentations, including occult/gross bleeding and emergencies (intussusception, obstruction, perforation); imaging recommended for early diagnosis. ^[3]
• Case series: common symptoms (pain, bleeding, obstruction), extensive use of CT, and role of surgical resection; melanoma can metastasize to GI even after early stages. ^[1]
• Small-bowel melanoma case reports: suspicion with bleeding/obstruction; role of capsule endoscopy, enteroscopy, CT, PET‑CT; surgical resection can improve quality of life. ^[9]
• Melanoma spread patterns to organs; GI mucosal melanoma sites and anorectal bleeding/pain as signs. ^{[4] [5] [6]}
• Immunotherapy patient guidance on urgent symptoms like uncontrolled diarrhea and dark, tarry stools. ^{[7] [8]}