There is no single cause of colorectal cancer, but some factors appear to increase the risk of developing it:
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age – particularly after 50
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having polyps (small growths on the inner wall of the colon and rectum)
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family history of colorectal cancer - especially if the relative (parent, sibling, child) developed colorectal cancer before the age of 45
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having familial adenomatous polyposis (FAP) or hereditary nonpolyposis colon cancer (HNPCC)
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inflammatory bowel disease (ulcerative colitis or Crohn’s disease)
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smoking
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physical inactivity
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obesity
Some people develop colorectal cancer without any of these risk factors. This is why general screening for colon cancer with a full colonoscopy is so important.