The Connection Between Colon Cancer and Thin Stools

A tumor obstructing the colon may be the culprit

Close up of pants around businessman's ankles in restroom
Getty Images/LWA/Sharie Kennedy

When it is caught early, colon cancer often does not cause symptoms. Over time, though, if a tumor is allowed to grow, it can start to cause problems within the bowel. Although there are many causes of thin stools, this symptom can be caused by a large tumor in the colon.

What Is a Normal Bowel Movement?

At the risk of being indelicate, there is no such thing as a universally normal bowel movement. In fact, your doctor really only cares about what is normal for you.

In other words, everyone’s stool size, color, smell, and consistency are different.

That being said, bowel movements are typically brown in color due to bile breakdown during digestion. Bowel movements are also smelly -- this is a sign that you have good bacteria in your digestive tract, which your body uses to break down ingested foods.

Overall, the characteristics of your bowel movements are determined by a number of factors including:

Changes in Bowel Movements

You may have a bowel movement twice a day or twice a week -- only you will know if your bowel movements have changed. Sudden changes in bowel movement frequency, color, shape, or consistency could have many causes, many of them benign (non-cancerous). Some common causes of non-cancerous bowel habit changes include:

  • Dietary changes
  • Bacteria in food

If you passed a bowel movement once daily for the last 10 years, decreasing stool frequency to four bowel movements in one month is a major personal bowel habit change that should be reported to your doctor.

Similarly, if you normally pass very large, firm bowel movements and begin passing small, thin or watery stools, this bowel habit change should be investigated.

What Is a Thin Stool?

Thin stools are described as "ribbon-like," "pencil-like," "thin," or "narrow" bowel movements. If you normally pass stools the size of bananas, starting to pass stools the size of pencils would indicate a bowel habit change. 

Referring to the anatomy of the colon, stool is initially more of a thick liquid than a solid when it enters the colon. As the stool moves lower into your colon towards your rectum and anus, it absorbs water and salt and becomes more formed. In fact, the stool should be completely formed (solid) as it reaches your descending and sigmoid colon, which lies on the left side of your abdomen. 

How Can Colon Cancer Cause Thin Stools?

In an otherwise healthy person, thin stools related to colon cancer are usually caused by a narrowing within the colon, also called a partial blockage of the colon. Think of your colon as a hollow tube, like a drinking straw. If a tumor is growing inside the colon, it can start to obstruct the flow of stool through your colon, just like a pea inside a drinking straw would block or slow the flow of fluids.

Left-sided colon cancers most frequently cause bowel movement-related symptoms of colon cancer, such as thin or bloody stools.

Cancers of the rectum can also cause partial and complete blockages of stool passage, as well as an uncomfortable feeling of urgency. This symptom can be caused by a growing tumor within the small rectal space, which places pressure on the nerves, making you feel as if you need to move your bowels.

Symptoms to Watch Out For

If you have noted bowel habit changes for a few days and are waiting to see your doctor, pay special attention to other symptoms and discuss them with your doctor.

In addition to thin stools, colon cancer can cause:

  • Changes in bowel movement frequency (for example, diarrhea or constipation)
  • Abdominal bloating and cramping
  • Rectal bleeding
  • Blood in the stool, which may cause dark stools
  • Weight loss that is unintentional
  • Loss of energy
  • Loss of appetite

What May Increase My Chances of Developing a Bowel Obstruction?

Several risk factors may increase your chances of having a partial or complete intestinal blockage with colon cancer. For instance, two treatment modalities for colon cancer, abdominal surgery, and radiation, may increase your chances of a bowel obstruction, due to the formation of adhesions or scar tissue around the colon.

In addition, there are medical conditions other than colon cancer that may cause a blockage in the colon and thin or absent stools. Examples include a hernia, diverticulitis, or a cancer of the stomach and ovary.

A Word From Verywell

Once you have identified bowel habit changes, the first step is to report these and any related symptoms to your doctor. He can order tests to rule out colon cancer and other, less serious causes. Remember, when it comes to your stool, you are the expert, so remain proactive in your bowel health. 


American Cancer Society. (2016). Colorectal Cancer Signs and Symptoms.

John SK, S George, JN Primrose, JB Fozard. Symptoms and signs in patients with colorectal cancer.Colorectal Dis. 2011 Jan;13(1):17-25.

National Cancer Institute. (2016). Gastrointestinal Complications - Patient Version: Bowel Obstruction.