The diagram here depicts a normal colon section and one bearing cancerous growths.

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In the United States, colorectal cancer—cancer of the colon (large intestine) and rectum, also called bowel cancer—is the third-most common type of cancer and the second-leading cause of cancer death. The vast majority of these cancers are adenocarcinomas, which usually develop from growths of tissue (polyps) known as adenomas on the inner lining of the colon or rectum.

The cancer tends to grow slowly, and it may take five to 10 years for a polyp to become cancerous. Trouble is, you may not experience any signs of colon cancer until the disease has progressed. As such, it’s important to undergo routine screening to detect and remove precancerous polyps well before you develop any bowel cancer symptoms. (See our post “Colorectal Cancers: Screening Saves Lives,” by Rosanne Leipzig, MD, PhD.)

Telltale Colon Cancer Symptoms

Oftentimes, patients experience few or no colon cancer symptoms until the disease is quite advanced and potentially has spread, which is why screening is so important. Some symptoms that should prompt you to see a doctor include:

• A change in your typical bowel habits, such as diarrhea or constipation, that lasts for more than several days.
• A change in stool diameter, especially a narrowing of your stool, that continues beyond several days.
• Blood in the stool, either bright red or very dark blood.
• A feeling that your bowel has not emptied completely.
• Unexplained weight loss.
• Abdominal pain or cramping.
• Fatigue.

These bowel cancer symptoms often are signs of a less serious illness, such as hemorrhoids, infection, irritable bowel syndrome, or inflammatory bowel disease (ulcerative colitis or Crohn’s disease). Nevertheless, since they could be colon cancer symptoms, be sure to have them checked by a physician right away.

Early Detection of Bowel Cancer Is Crucial

The fact that colorectal cancer most often begins as an adenoma makes it one of the most preventable cancers, as the pre-cancerous polyps can be detected and removed before they become malignant.

Most medical organizations, including the American College of Gastroenterology (ACG), recommend that everyone at average risk of colorectal cancer begin screening for the disease at age 50 (the ACG recommends that African-Americans, who generally have a higher risk of bowel cancer, start screening with colonoscopy at age 45).

Why to Get a Colonoscopy Screening

Colonoscopy is widely considered the “gold standard” screening method because it not only affords a visual inspection of the entire colon, but it also allows for the immediate removal of any polyps found during the exam. The ACG recommends the following screening options and intervals for average-risk patients to prevent colorectal cancer:

• Colonoscopy every 10 years (preferred option).
• Flexible sigmoidoscopy, an endoscopic inspection of the lower third of the colon, every five to 10 years.
• Computed tomography (CT) colonography, also known as virtual colonoscopy, every five years.
• Double-contrast barium enema every five years.

For patients who decline these prevention tests, the ACG recommends tests that detect cancer but not polyps:

• Annual fecal immunochemical test (FIT), which identifies blood in the stool and is the preferred colorectal cancer detection test.
• Annual high-sensitivity fecal occult blood test (Hemoccult Sensa).
• Fecal DNA test (interval uncertain).

Note that abnormal findings on any of the screening tests, besides colonoscopy, warrant a follow-up inspection with colonoscopy.

For higher-risk patients—those with a parent or sibling with colorectal cancer or advanced adenomas diagnosed before age 60, or two first-degree relatives with bowel cancer or advanced adenomas—the ACG recommends screening with colonoscopy every five years, beginning at age 40, or 10 years before the youngest age at which the family member developed the disease.

People with inflammatory bowel disease or inherited syndromes that place them at higher risk of colorectal cancer should be screened more frequently.

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Originally published in May 2016 and updated.