A colon polyp is a small clump of cells that forms on the lining of the colon. Although most colon polyps are harmless, some become cancerous over time.

Anyone can develop colon polyps. But you’re at higher risk if you’re 50 or older, are overweight or a smoker, eat a high-fat, low-fiber diet, or have a personal or family history of colon polyps or colon cancer.

Usually colon polyps don’t cause symptoms. That’s why experts recommend regular screening. Colon polyps that are found in the early stages usually can be removed safely and completely. Screening helps prevent colon cancer, a common disease that’s often fatal when it’s found in its later stages.

Colon polyps often cause no symptoms. You might not even know you have a polyp until your doctor finds it during an examination of your bowel. Sometimes, however, you may have signs and symptoms such as:

• Rectal bleeding. You might notice bright red blood in pan after passing stool. Although this may be a sign of colon polyps or colon cancer, rectal bleeding can indicate other conditions, such as hemorrhoids or minor tears (fissures) in your anus. You should discuss any rectal bleeding with your doctor.
• Blood in your stool. Blood can show up as red streaks in your stool or stool may appear black. Still, a change in color doesn’t always indicate a problem — iron supplements and some anti-diarrhea medications can make stools black, whereas beets can turn stools red. Always discuss any rectal bleeding with your doctor.
• Constipation, diarrhea or narrowing of the stool. Although a change in bowel habits that lasts longer than a week may indicate the presence of a large colon polyp, it can also result from a number of other conditions.
• Pain or obstruction. Sometimes a large colon polyp may partially obstruct your bowel, leading to crampy abdominal pain, nausea, vomiting and severe constipation.

The last part of your digestive tract is a long muscular tube called the large intestine. The colon makes up most of the large intestine. The rectum and anus make up the end of the large intestine. The colon’s main function is to absorb water, salt and other minerals from colon contents. Your rectum stores waste until it’s eliminated from your body as stool.

The majority of polyps aren’t cancerous (malignant). Yet like most cancers, polyps are the result of abnormal cell growth. Healthy cells grow and divide in an orderly way — a process that’s controlled by two broad groups of genes. Mutations in any of these genes can cause cells to continue dividing even when new cells aren’t needed. In the colon and rectum, this unregulated growth can cause polyps to form. Over a long period of time, some of these polyps may become malignant.

Polyps can develop anywhere in your large intestine. They can be small or large and flat (sessile) or mushroom shaped and attached to a stalk (pedunculated). In general, the larger a polyp, the greater the likelihood of cancer.

• About two-thirds of all polyps fall into this category. Although only a small percentage of these polyps actually become cancerous, nearly all malignant polyps are adenomatous.
• Most of the remaining polyps are hyperplastic. These polyps occur most often in your left (descending) colon and rectum. Usually less than a quarter of an inch (5 millimeters) in size, they’re very rarely malignant.
• These polyps may follow a bout of ulcerative colitis or Crohn’s disease of the colon. Although the polyps themselves are not a significant threat, having ulcerative colitis or Crohn’s disease of the colon increases your overall risk of colon cancer.

A number of factors may contribute to the formation of colon polyps and colon cancer. They include:

• Most people with colon polyps are 50 or older.
• Inflammatory intestinal conditions. Long-standing inflammatory diseases of the colon, such as ulcerative colitis and Crohn’s disease, can increase your risk.
• Family history. You’re more likely to develop colon polyps or cancer if you have a parent, sibling or child with them. If many family members have them, your risk is even greater. In some cases this connection isn’t hereditary or genetic. For example, cancers within the same family may result from shared exposure to a cancer-causing substance (carcinogen) in the environment or from similar diet or lifestyle factors.
• Tobacco and alcohol use. Smoking significantly increases your risk of colon polyps and colon cancer. Drinking alcohol, especially beer, in excess also makes it more likely that you’ll develop colon polyps.
• A sedentary lifestyle. If you’re inactive, you’re more likely to develop colon cancer. This may be because when you’re inactive, waste stays in your colon longer.
• Being overweight or obese has been linked to an increased risk of several types of cancer, including colon cancer.

Another risk factor for colon polyps is genetic mutations. A small percentage of colon cancers result from gene mutations. Some of these cancers are autosomal dominant, meaning you need to inherit only one defective gene from either one of your parents. If one parent has the mutated gene, you have a 50 percent chance of inheriting the mutation. Although inheriting a defective gene greatly increases your risk, not everyone with a mutated gene develops cancer.

• Familial adenomatous polyposis (FAP). This is a rare, hereditary disorder that causes you to develop hundreds, even thousands, of polyps in the lining of your colon beginning during your teenage years. If these go untreated, your risk of developing colon cancer is nearly 100 percent, usually before age 40. People with FAP are also at risk of cancers of the small intestine, particularly in the duodenum.
• Gardner’s syndrome. This less common syndrome is a variant of FAP. This condition causes polyps to develop throughout your colon and small intestine. You may also develop noncancerous tumors in other parts of your body, including your skin (sebaceous cysts and lipomas), bone (osteomas) and abdomen (desmoids).
• Lynch syndrome. This condition, also called hereditary nonpolyposis colorectal cancer (HNPCC) is the most common form of inherited colon cancer. People with Lynch syndrome tend to develop relatively few colon polyps, but those polyps can quickly become malignant. Or, people with Lynch syndrome may have tumors in other organs, including the breast, stomach, small intestine, urinary tract and ovary, as well as in the colon.
• Peutz-Jeghers syndrome (PJS). This genetic condition usually begins with freckles developing all over the body, including the lips, gums and feet. Then benign polyps develop throughout the intestines. These polyps may become malignant, so people with this condition have an increased risk of colon cancer.

Some colon polyps may become cancerous (malignant). The earlier polyps are removed, the less likely it is that they will become malignant.

Nearly all colon cancers develop from polyps, but the polyps grow slowly, usually over a period of years. Screening tests play a key role in detecting polyps before they become cancerous. These tests can also help find colorectal cancer in its early stages, when you have a good chance of recovery.

Several screening methods exist — each with its own benefits and risks. Be sure to discuss these with your doctor:

• Colonoscopy is performed with a long, slender, flexible tube attached to a video camera and monitor. During colonoscopy, your doctor typically views your entire colon and rectum.

This procedure is the most sensitive test for colorectal polyps and colorectal cancer. If any polyps are found during the exam, your doctor may remove them immediately or take tissue samples (biopsies) for analysis. In order to prepare for the exam, you follow your doctor’s instructions on restricting your diet and taking laxatives to cleanse your bowel. You’re likely to receive a mild sedative to make you more comfortable. The risks of diagnostic colonoscopy include hemorrhage and perforation of the colon wall. Complications are more likely to occur when polyps are removed.

• Computerized tomographic colonography (CTC). Also referred to as virtual colonoscopy, this test involves a computerized tomography scan, a highly sensitive X-ray of your colon. Using computer imaging, your doctor rotates this X-ray in order to view every part of your colon and rectum without actually going inside your body. Before the scan, your large intestine is cleared of any stool, but researchers are looking into whether the scan can be done successfully without the usual bowel preparation.

This newer technology may make colon screening safer, more comfortable and less invasive. It can be done more quickly and doesn’t require sedation. However, it may not be as accurate as regular colonoscopy. Also, this method doesn’t allow your doctor to remove polyps or take tissue samples during the procedure. If your doctor finds polyps or wants to sample tissue, you will need a colonoscopy.

• Flexible sigmoidoscopy. In this test, your doctor uses a slender, lighted tube to examine your rectum and sigmoid — approximately the last two feet (61 centimeters) of your colon. Nearly half of all colon cancers are found in this area. If your doctor finds a polyp during this test, you’ll need a colonoscopy so that your doctor can see your entire colon and remove any polyps.

A sigmoidoscopy looks at only the last third of your colon, and this test doesn’t detect polyps elsewhere in the large intestine. A sigmoidoscopy can be somewhat uncomfortable. Also, there’s a slight risk of perforating the colon, but the risks are less than they are for colonoscopy.

• Barium enema. This diagnostic test allows your doctor to evaluate your entire large intestine with an X-ray. A contrast solution containing barium is placed into your bowel in enema form. The barium fills and coats the lining of the bowel, creating a silhouette of your rectum, colon and sometimes a small portion of your small intestine. Air also may be added to provide better contrast on the X-ray.

The image produced with the barium enema test isn’t as detailed as other screening methods and polyps may be missed on this exam. It also doesn’t allow your doctor to take a biopsy during the procedure to determine whether a polyp is cancerous. This test can be somewhat uncomfortable because the barium and air stretch your bowel. There’s also a slight risk of perforating the colon wall.

Although some types of colon polyps are far more likely to become malignant than are others, a pathologist usually must examine polyp tissue under a microscope to determine whether it’s potentially cancerous. For that reason, your doctor is likely to remove all polyps discovered during a bowel examination.

The great majority of polyps can be removed during colonoscopy or sigmoidoscopy by snaring them with a wire loop that simultaneously cuts the stalk of the polyp and cauterizes it to prevent bleeding. Some small polyps may be cauterized or burned with an electrical current. Risks of polyp removal (polypectomy) include bleeding and perforation of the colon.

Polyps that are too large to snare or that can’t be reached safely are usually surgically removed — often using laparoscopic techniques. This means your surgeon performs the operation through several small incisions in your abdominal wall, using instruments with attached cameras that display your colon on a video monitor. Laparoscopic surgery may result in a faster and less painful recovery than does traditional surgery using a single large incision. Once the section of your colon that contains the polyp is removed, the polyp can’t recur, but you have a moderate chance of developing new polyps in other areas of your colon in the future. For that reason, follow-up care is extremely important.

Some specialized medical centers perform endoscopic mucosal resection (EMR) to remove larger polyps with a colonoscope. For this newer technique a liquid, such as saline, is injected under the polyp to elevate and isolate the polyp from surrounding tissue. This makes it easier to remove a larger polyp. With this procedure, you can avoid surgery, but it’s not yet clear how the complication rates may compare.

In cases of rare, inherited syndromes, such as familial adenomatous polyposis (FAP), your surgeon may perform an operation to remove your entire colon and rectum (total proctocolectomy). Then, in a procedure known as ileal pouch-anal anastomosis, a pouch is constructed from the end of your small intestine (ileum) that attaches directly to your anus. This allows you to expel waste normally, although you may have watery and more frequent bowel movements.