Colorectal cancer causes and risk factors

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on September 13, 2022.

Colorectal cancer occurs when cells grow out of control anywhere in the colon or the rectum, which are among the last stops for food in the digestive tract.

The large intestine, which is about 5 feet long, includes the colon, which connects to the anus. The overall length of this impressive digestive system is a lot of ground for potential cellular changes. Colorectal cancer most often forms in the mucus-making cells of the colon and rectum.

Understanding the risk factors for colorectal cancer is key to prevention, since taking proactive measures may reduce the likelihood of developing the disease. Patients with a higher risk should consult their doctor about when to get a colonoscopy or a stool test, which may help spot the early symptoms of colorectal cancer.

What causes colorectal cancer?

Colorectal cancer forms when the DNA in cells in the colon or rectum develop mutations that may make them unable to control growth and division. In many cases, these mutated cells die or are attacked by the immune system. But some mutated cells may escape the immune system and grow out of control, forming a tumor in the colon or rectum.

The exact cause of colorectal cancer is not known, but certain risk factors are strongly linked to an increased risk of developing the disease:

  • Diet
  • Tobacco use
  • Smoking
  • Heavy alcohol use

Also, people with certain hereditary cancer syndromes or a family history of colorectal cancer have a high risk of developing the disease.

Download the colorectal cancer infographic >>

Why colorectal cancer screening matters

A screening test is a proactive way to look for signs of cancer or other conditions. Through screening, the doctor may find polyps before they turn into cancer. If found early enough, cancer is often easier to treat.

Doctors develop a colorectal cancer screening plan based on personal risk factors and family history. Screening options for colorectal cancer include:

  • Lab test: A sample of stool is examined for traces of blood
  • Flexible sigmoidoscopy: A tube with a tiny camera on the end is inserted into the rectum to examine the rectum and lower part of the colon
  • Colonoscopy: A tube with a tiny camera on the end is inserted into the rectum to examine the rectum and entire colon
  • Computed tomography (CT) colonography, also known as a virtual colonoscopy: Takes images of the entire colon with combined power from X-rays and computers

Colorectal cancer risk factors patients can't control

Some things in life just can’t be controlled, and that’s the same for some characteristics—but not all—when it comes to colorectal cancer risk.

These characteristics may increase the risk of developing colorectal cancer:

  • Age—Colorectal cancer cases are increasing in people younger than age 64, and increasing even faster for those younger than 50, so education and prevention remain critical. According to the Centers for Disease Control and Prevention, about 90 percent of colorectal cancer cases are diagnosed in people aged 50 or older.
  • Race and ethnicity—Colorectal cancer diagnosis and deaths are highest among non-Hispanic African Americans. Those rates are also higher in American Indians, Alaska Natives and Jewish people of eastern and central European descent (Ashkenazi).
  • History of polyps or cancer—The risk is higher for those who’ve previously had colorectal polyps, especially if these were large, copious or had abnormal-but-noncancerous cells (dysplasia). The risk is also higher for a patient who’s had colorectal cancer before.
  • Certain health conditions—Inflammatory bowel disease (IBD, including ulcerative colitis or Crohn's disease), Gardner syndrome or type 2 diabetes may raise the risk of developing colorectal cancer. Irritable bowel syndrome (IBS) doesn’t seem to be linked to a higher risk.
  • Certain genetic syndromes—Having an inherited condition may lead to an increased risk of colorectal cancer. These include Lynch syndrome or polyposis syndromes such as familial adenomatous polyposis (FAP). Researchers are studying whether other gene mutations, like the BRCA1 and BRCA2 mutations linked to breast cancer, may also increase the potential risk for developing colorectal cancer.
  • Family health history—If other family members have had colorectal cancer or colon polyps, the risk of developing colorectal cancer is higher.

Controllable colorectal cancer risk factors

According to the American Cancer Society (ACS), 55 percent of colorectal cancer diagnoses in the United States are attributed to controllable lifestyle factors, including those below.

  • Diet: The risk is higher for those who eat a high-fat diet, a lot of processed meat or red meat. Aim for a healthy diet rich in fruits, whole grains, vegetables and fiber.
  • Drinking: Moderate to heavy alcohol consumption, and even light to moderate drinking, may raise the risk of colorectal cancer. Men should limit themselves to two drinks a day and women to one drink a day.
  • Weight: The risk for colorectal cancer is higher for people who are overweight or obese, especially for men.
  • Activity level: There’s a connection between physical activity and colon cancer, but not rectal cancer. People who spend a lot of the day sitting or lying down may be more likely to develop colorectal cancer.
  • Smoking: According to an ACS report, the International Agency for Research on Cancer has said definitively that smoking tobacco causes colorectal cancer. About 12 percent of U.S. colorectal cancer cases are caused by current or former smoking.
  • Preventive medications: Taking aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs) regularly and for a long-term period has been shown to decrease the risk of colorectal cancer, especially for people younger than 70 who are at a healthy weight. Patients should consult first with a doctor before using these medications.

How common is colorectal cancer?

Colorectal cancer is the fourth-most common cancer in the United States, according to the National Cancer Institute Surveillance, Epidemiology, and End Results Program. It’s also the second deadliest out of cancers that affect both men and women.

While those numbers may sound overwhelming, the ACS states that an estimated 4.1 percent of all men and women are expected to be diagnosed with colorectal cancer in their lifetime.

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