Colon cancer can cause poop (stool) changes such as blood in the stool and stools that are looser and more frequent. People with colon cancer may also have other symptoms such as abdominal pain.

These symptoms are common in other gastrointestinal conditions and don’t necessarily mean you have colon cancer. It is also important to remember that some people do not have any symptoms, and it is possible to have healthy/normal-looking poop with colon cancer.¹

This article will help you understand the relationship between colon cancer and poop, what signs to look for, and what to do if you recognize symptoms of colon cancer.

Colon Cancer and Poop: Early Signs

One of the first signs of colon cancer is a change in poop or in bowel habits. Stool with colon cancer can look thinner and have a different consistency and color than typical stools.

Arguably, more than any other symptom, unexplained changes in the appearance or movement of stool should raise concerns about colon cancer, particularly if the symptoms persist or worsen.

Some of the subtle warning signs of colon cancer in the early stages can include:

• Narrow “pencil” stools: Caused by the narrowing of the intestinal passage as the tumor starts to grow
• Pain with defecation: Also known as dyschezia, a symptom commonly associated with rectal cancer
• A sensation of being unable to empty the bowel: Also known as tenesmus, commonly seen with early-stage rectal cancer
• Rectal bleeding: More common with rectal cancer or cancer in the lower bowel
• Abdominal pain: Caused when localized inflammation triggers pain receptors called visceral nociceptors
• Fatigue: Due to intestinal bleeding, the loss of red blood cells, and the development of anemia²

Poop Changes in Regional Colon Cancer

In stage 3 colon cancer, the malignancy has spread to nearby lymph nodes. In this stage, changes in poop and bowel habits tend to become more apparent as the intestinal passageway (called the lumen) further narrows and the tumor starts to grow into deeper layers of the intestinal tissues.

The progression of colon cancer can often be recognized by the following additional symptoms:

• Constipation: Due in part to increasing bowel obstruction
• Diarrhea: Caused by an overflow of intestinal fluid around a partial obstruction
• Alternating constipation and diarrhea: A more telling sign of colon cancer
• Blood in stool: Also known as hematochezia, which can be bright red if the tumor is near the rectum or dark red or maroon if higher up in the colon
• Tenesmus: Can occur as colon cancer progresses due to the persistent and increasing inflammation
• Abdominal bloating and cramps: Due in part to the inability to pass gas when the bowel is obstructed
• Symptoms of iron deficiency anemia: Occurs in around 48% of people with colon cancer³

Wilson MJ, Dekker WT, Harlaar JJ, Jeekel J, Schipperus M, Zwaging JJ. The role of preoperative iron deficiency in colorectal cancer patients: prevalence and treatment. Int J Colorectal Dis. 2017;32(11):1617-24. doi:10.1007/s00384-017-2898-1

The development of scar tissue and strictures can eventually cause bowel obstruction, either partial or complete. Bleeding tends to be greater, although it can’t always be seen in the stool itself.

Does poop smell different with colon cancer?

Foul-smelling stool can be a sign of a medical problem,⁴ but because there are many possible conditions that can cause this, it is not possible to identify colon cancer based on stool odor. Researchers are working on technology that can identify colon cancer based on the odor of stool, but these types of changes can be too subtle for human noses to detect.⁵

Poop Changes in Advanced Colon Cancer

Stage 4 colon cancer is characterized by the spread of cancer to distant organs (also referred to as metastasis). It is the most advanced stage of cancer and one that can manifest with more overt symptoms as the obstruction and bleeding of the bowel worsens.

In this stage, poop may look black and tarry. Also known as melena, this is more often seen with right-sided cancer. It happens as hemoglobin in the blood reacts with oxygen during its long passage toward the rectum.⁶

Other symptoms can include:

• Nausea and vomiting: Generally seen when obstruction becomes severe
• Unexplained weight loss: A common consequence of advanced cancer in which systemic inflammation triggers muscle and weight loss as well as the loss of appetite⁷

In some cases, bowel perforation can occur as the tissues are stressed and eventually rupture. Other symptoms can develop due to the spread of cancer to the liver, chest, bone, and other common sites of metastasis.⁸

Although people with stage 4 colon cancer are more likely to develop symptoms, not everyone does. In fact, in the absence of obstruction, perforation, or active bleeding, many people with metastatic colon cancer can be entirely asymptomatic.⁹

When to Call 911

Call 911 or seek emergency care if you develop the following:

• Heavy rectal bleeding
• Severe abdominal pain and cramping
• Rigid abdominal muscles
• Fever and chills
• Vomiting
• Cool, clammy skin
• Rapid heartbeat

These could be signs of peritonitis, a potentially life-threatening condition associated with intestinal perforation.

Summary

Colon cancer can cause changes in the color, consistency, and frequency of your poop. These changes can include blood in the stool, narrow stools, more frequent bowel movements, and diarrhea. In advanced stages, poop may become black and tarry.

Changes in bowel habits and the consistency of poop can also happen with other gastrointestinal conditions. Though these symptoms aren’t always a sign of cancer, you should always have them checked out by a healthcare provider. Colon cancer is highly treatable if it’s caught in the early stages.

Important Notice: This article was originally published at www.verywellhealth.com by James Myhre & Dennis Sifris, MD where all credits are due. Medically reviewed by Doru Paul, MD

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