Ulcers in the colon form when the inner lining of the large intestine becomes damaged, exposing the tissue beneath. The most common causes are inflammatory bowel disease, infections, reduced blood flow, and certain medications. Each of these triggers damage through a different mechanism, and identifying the cause is essential because treatment varies widely depending on what’s driving the ulceration.
Inflammatory Bowel Disease
Inflammatory bowel disease (IBD) is the most well-known cause of colonic ulcers, and it comes in two main forms: ulcerative colitis and Crohn’s disease. Both involve the immune system attacking the colon’s lining, but they do so in distinct patterns.
In ulcerative colitis, the immune system floods the colon wall with white blood cells that release waves of inflammatory signals. These signals recruit even more immune cells, creating a self-reinforcing cycle of inflammation. One key protein, TNF-alpha, directly kills the cells lining the colon and breaks apart the tight junctions between them. Other inflammatory signals interfere with the colon’s ability to repair itself, which is why the damage persists and worsens over time. Ulcers in ulcerative colitis typically start small and scattered, then merge into larger linear or circular ulcers as the disease progresses. The damage almost always begins in the rectum and spreads upward in a continuous pattern.
Crohn’s disease can affect any part of the digestive tract, but when it involves the colon, its ulcers look quite different. They often begin as small, shallow sores called aphthous ulcers and can deepen significantly into the bowel wall. Crohn’s ulcers tend to appear in patches, with healthy tissue between affected areas. When viewed during a colonoscopy, they sometimes create a “cobblestone” appearance where inflamed and swollen tissue alternates with deep grooves.
IBD prevalence has been climbing. A 2024 population study from New Zealand found ulcerative colitis rates had nearly doubled over two decades, rising from 145 to 264 per 100,000 people. Some regions report even higher figures, with parts of Denmark recording ulcerative colitis rates above 900 per 100,000.
Infections That Damage the Colon
Several bacteria, viruses, and parasites can directly invade and ulcerate the colon lining. The culprit matters because each infection requires different treatment.
Clostridioides difficile (C. diff) is the most common bacterial cause. It typically strikes after a course of antibiotics wipes out the colon’s normal protective bacteria, giving C. diff room to multiply. The toxins it produces destroy the surface cells and trigger severe inflammation, sometimes forming a characteristic layer of debris over the ulcerated tissue called a pseudomembrane. Less commonly, bacteria like Klebsiella oxytoca and Staphylococcus aureus cause similar damage.
Cytomegalovirus (CMV) is a particular concern for people with weakened immune systems, including those already being treated for ulcerative colitis. CMV can infect the colon and cause deep ulcers that mimic an IBD flare. When someone with ulcerative colitis stops responding to treatment, CMV superinfection is one of the first things doctors investigate.
Parasites like Entamoeba histolytica, the organism behind amoebic dysentery, can burrow into the colon wall and produce flask-shaped ulcers. This is more common in tropical regions and among travelers returning from endemic areas.
Reduced Blood Flow (Ischemic Colitis)
The colon needs a steady supply of blood to stay healthy. When that supply drops, the oxygen-starved tissue begins to break down, and ulcers form. This is called ischemic colitis, and it primarily affects people over 60 who have underlying heart disease or hardened arteries.
The blood flow reduction can be sudden or gradual. Sometimes a triggering event, like a drop in blood pressure during surgery, dehydration, or a cardiac event, pushes an already marginal blood supply below the threshold the colon tissue needs to survive. The first signs are swelling and small hemorrhages in the colon wall, which can progress to full tissue death and deep, segmental ulceration if the ischemia is severe enough.
Certain areas of the colon are more vulnerable because they sit at the boundaries between major arterial supply zones. The splenic flexure (where the colon bends near the spleen) and the junction between the sigmoid colon and rectum are classic trouble spots. Mild cases heal on their own once blood flow is restored, but severe ischemia can lead to tissue death that requires surgery.
Medications
Non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen are a well-established but often overlooked cause of colonic ulcers. Most people associate these drugs with stomach ulcers, but they can damage the colon too. NSAIDs work by blocking the production of prostaglandins, which are chemicals that help regulate inflammation. The problem is that prostaglandins also maintain the protective mucus layer in the colon and support blood flow to the intestinal lining. When that protection disappears, the colon becomes vulnerable to acid, bacteria, and mechanical stress.
NSAID-related colon ulcers can sometimes form raised, mass-like lesions that mimic tumors on imaging, leading to unnecessary concern about cancer until a biopsy clarifies the diagnosis. Radiation therapy to the pelvis, used to treat cancers of the prostate, cervix, or rectum, can also damage the colon lining and produce chronic ulceration known as radiation colitis.
Behcet’s Disease and Other Rare Causes
Behcet’s disease is a rare condition that inflames blood vessels throughout the body and can produce distinctive ulcers in the colon. Unlike the elongated ulcers of Crohn’s disease, Behcet’s colitis tends to create round, deep, “punched-out” ulcers, most often near the junction of the small and large intestine. The underlying mechanism is a type of blood vessel inflammation called venulitis, where immune cells attack the small veins in the colon wall. Behcet’s is notoriously difficult to distinguish from Crohn’s disease and intestinal tuberculosis, and diagnosis often depends on the presence of other Behcet’s symptoms like recurrent mouth sores, genital ulcers, and eye inflammation.
Other systemic conditions that can occasionally cause colonic ulcers include vasculitis (inflammation of blood vessels from various autoimmune diseases) and certain blood disorders that compromise the colon’s oxygen supply.
How Colonic Ulcers Are Identified
Because so many different conditions produce ulcers in the colon, a colonoscopy with tissue sampling is the standard approach for sorting out the cause. Doctors look at several features: the shape and depth of the ulcers, whether they’re continuous or patchy, where in the colon they appear, and what the surrounding tissue looks like. A biopsy, where a small piece of tissue is removed during the colonoscopy for examination under a microscope, is almost always necessary. No single visual finding is enough to confirm a diagnosis on its own.
The pattern of ulceration often provides the first strong clue. Continuous inflammation starting from the rectum points toward ulcerative colitis. Patchy, deep ulcers with normal tissue in between suggest Crohn’s disease. Ulcers confined to a segment of colon in an older patient with heart disease raise suspicion for ischemia. And ulcers that develop shortly after antibiotic use prompt testing for C. diff.
Symptoms to Recognize
Colonic ulcers share a core set of symptoms regardless of the underlying cause. The most common are diarrhea, blood in the stool (which may appear bright red or dark, depending on the ulcer’s location), and abdominal pain or cramping. The severity ranges widely. Some people notice only occasional streaks of blood on toilet paper, while others experience urgent, frequent bloody diarrhea with significant cramping.
Weight loss, fatigue, and fever can accompany more severe or prolonged ulceration. Ischemic colitis often presents with sudden abdominal pain on the left side followed by bloody diarrhea within hours. Infectious causes tend to come on acutely with watery or bloody diarrhea and sometimes fever. IBD-related ulcers more often follow a pattern of flares and remissions that develop over weeks to months.
Serious complications, while uncommon, include heavy bleeding, perforation (a hole through the colon wall), and in severe ulcerative colitis, toxic megacolon, where the colon dilates dangerously. These are medical emergencies that require immediate treatment.