Medically Reviewed on 7/11/2023

What is colitis?

Colitis describes inflammation of the inner lining of the colon and can be associated with diarrhea, abdominal pain, bloating, and blood in the stool. This inflammation may be due to a variety of reasons, including the following:

Colitis and the anatomy of the colon

The colon, or large intestine, is a hollow, muscular tube that processes waste products of digestion from the small intestine, removes water, and ultimately eliminates the remnants as feces (stool) through the anus. The colon is located within the peritoneum, the sac that contains the intestine, located in the abdominal cavity.

The colon is surrounded by many layers of tissue. The innermost layer of the colon is the mucosa that comes into contact with the waste products of digestion. The mucosa absorbs water and electrolytes back into the blood vessels that are located just below the surface in the submucosa. This is surrounded by a circular layer of muscles and then another outer layer of longitudinal muscles that run along the length of the colon. The muscles work together to rhythmically squeeze liquid waste from the cecum through the entire length of the colon. Water is gradually removed, turning the waste into the formed stool so that it is excreted out of the anus in solid form.

The colon frames the organs within the peritoneum, and its segments are named based on their location.

  • The colon usually begins in the right lower quadrant of the abdomen, where the terminal ileum, the last part of the small intestine, attaches to the cecum, the first segment of the colon. The appendix is attached to the cecum.
  • The ascending colon begins at the cecum and arises from the right lower abdomen to the right upper abdomen near the liver.
  • The colon then makes a sharp left turn called the hepatic flexure (hepatic=liver) and is referred to as the transverse colon, as it makes its way to the left upper quadrant of the abdomen near the spleen.
  • There is a sharp downward turn called the splenic flexure, and it is referred to as the descending colon as it runs from the left upper quadrant to the left lower quadrant of the abdomen.
  • When it descends into the pelvis, it is referred to as the sigmoid colon.
  • The last several centimeters of the colon is referred to as the rectum.
  • The anus is the final portion of the colon.

Microscopic colitis

There are two types of microscopic colitis:

  1. Collagenous colitis
  2. Lymphocytic colitis

Either collagen or lymphocytes (a type of white blood cell) infiltrate into the layers of the wall of the colon, presumably as a result of inflammation. This is an uncommon illness and maybe an autoimmune disease. Diarrhea often is watery, and no blood is present in the stool.

Allergic colitis in infants

In infants younger than 1 year of age, colitis is often due to allergies to cow or soy milk. Allergic colitis may be seen in breastfed babies, where mothers drink cow's milk and pass that protein into their breast milk.

What are the five causes (types) of colitis?

Colitis describes inflammation of the colon (col=colon + itis=inflammation).

Examples of causes (types) of colitis include:

  1. Infection, for example, caused by bacteria like C. difficile, viruses, and parasites
  2. Inflammatory bowel diseases like Crohn's disease and ulcerative colitis
  3. Ischemic colitis caused by decreased blood supply
  4. Microscopic colitis (lymphocytic/collagenous colitis)
  5. Allergic reactions

Infectious causes of colitis

Many bacteria reside in the colon; they live in harmony with the body and cause no symptoms. However, some infections can result if a virus, bacteria, or parasite invades the small and/or large intestine.

Common bacteria that cause colitis include:

These infections usually occur because the patient has eaten contaminated food. Symptoms can include diarrhea with or without blood, abdominal cramps, and dehydration from water loss because of numerous watery, bowel movements. Other organs can also be affected by the infection or the toxins that the bacteria can produce.

Clostridium difficile, commonly referred to as C. diff, is a bacterial cause of colitis that often occurs after a person has been prescribed an antibiotic or has been hospitalized. C. diff is found in the colon of healthy people and coexists with other "normal" bacteria. But when antibiotics are prescribed, susceptible bacteria in the colon can be destroyed, allowing the clostridia to grow unchecked, causing colitis. Patchy membranes form over the colon mucosa and some health care professionals refer to C. diff colitis as pseudomembranous colitis. The bacteria also may be found on many surfaces in the hospital (for example, bedrails, toilets, and stethoscopes), and the infection may spread from person to person (it is highly contagious). Unfortunately, this infection is becoming more common outside the hospital environment, and people can develop community-acquired C. diff colitis without exposure to antibiotics or a medical facility.

Worldwide, the most common parasite infection to cause colitis is Entamoeba histolytica. It is acquired by drinking infected water and can also be passed from person to person because of poor sanitation and hygiene.


Ulcerative Colitis: Symptoms, Diet, Treatment, Causes See Slideshow

Ischemic causes of colitis

The colon can be thought of as a hollow muscle. It requires a normal blood supply to deliver oxygen and nutrients for the muscle to function normally. When the colon loses its supply of blood and becomes ischemic (isch= restricted + emia=blood supply), it may become inflamed. Ischemia or lack of blood supply causes inflammation of the colon leading to pain, fever, and diarrhea (bowel movements may contain blood).

Inflammatory bowel disease (IBD) and colitis

Ulcerative colitis and Crohn's disease are the two types of inflammatory bowel disease (IBD) that cause colitis. Crohn's disease and ulcerative colitis are considered autoimmune diseases (the body's immune system "attacks" itself).

  • Ulcerative colitis always begins in the rectum and may spread to the rest of the colon, spreading from the rectum to the sigmoid, descending, transverse, and finally the ascending colon and cecum in that order. Ulcerative colitis is considered an autoimmune disease, and symptoms include abdominal pain, and bloody, diarrheal bowel movements.
  • Crohn's disease may occur anywhere in the gastrointestinal tract (GI), including the esophagus, stomach, small intestine, and colon. In Crohn's disease, there may be "skip lesions," that is, abnormal segments of the GI tract interspersed with normal segments.

Both Crohn's disease and ulcerative colitis may have other organ systems involved in addition to the gastrointestinal tract.

What are the signs and symptoms of colitis?

Pain from inflammation

Inflammation of the colon causes the muscle layers to go into intermittent spasms, causing colicky or cramp-like pain that comes and goes. The pain usually is in the lower abdomen but can be felt anywhere along the course of the colon. Since the muscles fail to contract in a normal pattern and the colonic contents move through the colon rapidly, there is little opportunity for water to be reabsorbed. This leads to watery diarrhea. If the lining of the colon becomes inflamed and breaks down, bleeding may occur. In ulcerative colitis, small ulcers form and are the cause of bleeding.

Pain before, during, and/or after an episode of diarrhea

With colitis, particularly colitis involving the distal colon (rectum and sigmoid colon), the pain often crescendos and precedes a diarrheal bowel movement. After the bowel movement, the pain may subside but then returns with the next episode of diarrhea. The person also may have a feeling of urgency to have a bowel movement.

Other symptoms of colitis

Depending upon the cause of the colitis, other organ systems in the body may be involved and produce symptoms such as:

Both ulcerative colitis and Crohn's disease may have associated symptoms outside of the colon due to the body itself attacking other organs. These may include:

When should someone contact a doctor about colitis?

Diarrhea is a common sign of colitis. It is usually self-limited and resolves on its own with supportive care, including rest and a short course of a clear-fluid diet. However, seek medical care if diarrhea persists for more than two to three weeks, if there is blood in the stool, fever, or if the person has signs of dehydration.

  • Blood in the stool is never normal and should always be evaluated. Common causes of blood in the stool include hemorrhoids; however, other serious causes of bleeding need to be investigated. Colitis is not the only cause of rectal bleeding. Other causes include diverticular disease of the colon (diverticulitis), colon polyps, anal fissures, and cancer.
  • Chronic diarrhea may lead to dehydration and changes in the electrolyte balance in the body. If it is severe enough, the dehydration may require treatment with IV fluids or oral rehydration therapy. The symptoms of dehydration may include:
  • A high fever associated with diarrhea may be a warning sign that a significant infection may be present.
  • Abdominal pain is not normal, and while diarrhea may be associated with mild cramps, the presence of increasing abdominal pain requires the need to seek prompt medical attention.

What kind of doctor diagnoses and treats colitis?

Most often colitis is treated by a primary health care provider or internal-medicine specialist, especially when the colitis is first diagnosed and the cause is uncertain. Once the cause is found, that healthcare provider may be the only person needed to continue care. Should there be a need for further investigation and diagnostic testing, specialists may be consulted, such as a gastroenterologist. If an infection is the cause of colitis, an infectious-disease specialist may be consulted. In some cases, other specialists may be involved in treatment, such as surgeons and interventional radiologists.

Patient history

  • In patients with abdominal pain and diarrhea, it is important to find out when the symptoms began, how long they have lasted, whether they come and go, and what makes them better or worse.
  • Travel history is important, especially if the patient has recently visited an area with a potentially contaminated water supply or poor food hygiene. Patients often ask if the symptoms are caused by food poisoning, but that is a difficult question to answer immediately. Usually, this occurs with the consumption of poorly handled or stored food in a home or a family gathering event and symptoms occur within a few hours after the food was eaten. Moreover, others at the gathering may experience similar symptoms.
  • Blood in the stool, whether it is mixed in with the bowel movement or just drops in the toilet bowl, is not normal. While it may be due to hemorrhoids, other potential causes that are more worrisome may need to be explored. Questions might be asked about bowel habits, weight loss, weakness, or family history of bowel disorders, including cancer or colon polyps. Depending on the health care professional's concerns, information may need to be obtained about other body systems, past medical history, and social habits (including smoking, drinking, and occupational hazards or risks).

What happens during a physical exam for colitis?

Once the history is taken, a physical examination will help determine the potential causes of the symptoms.

  • Signs of more severe disease with dehydration may include orthostatic changes in blood pressure and pulse rate, where vital signs are taken both laying down and standing up. In dehydrated patients, who have had rectal bleeding, or are anemic, blood pressure and pulse may be normal when they are taken when the patient lies flat but may change when standing -- the blood pressure falls and the pulse rate rises. Prescription medications, especially beta-blockers prescribed for high blood pressure, may falsely lower the pulse rate and not allow it to increase.
  • Temperature is checked for fever.
  • Examination of the abdomen includes palpating or feeling for tenderness and masses in the abdomen. Bowel sounds are often listened to with a stethoscope.
  • The exam also may include a rectal examination to test the stool for blood and feel for a possible rectal mass.
  • If there is a concern for ischemic bowel as the cause of colitis, the examination also may assess the heart and blood vessels, looking for signs of atherosclerosis or narrowing of the arteries. This might include listening to the heart and palpating or feeling pulses in the neck, arms, and legs and listening with a stethoscope to the large blood vessels in the body including the carotid and femoral arteries and the aorta.
  • In patients where the clinical diagnosis of colitis is secondary to a viral infection, no further testing may be needed. However, this would not apply to a patient who appears ill, dehydrated or has significant pain, fever, or blood in the stool.

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What blood tests and/or stool samples diagnose colitis?

  • A complete blood count (CBC) measures hemoglobin and hematocrit, looking for anemia. If the red blood cell count is elevated, it may be due to dehydration, where total body water is decreased and the blood becomes concentrated.
  • The CBC also measures the white blood cell count, which may be elevated as the body responds to infection. However, an elevated white blood cell count does not necessarily equal infection, since elevation may be due to the body's reaction to any stress or inflammation.
  • Electrolytes may be measured by looking for changes in the sodium, potassium, chloride, and bicarbonate levels in the blood that help determine the severity of dehydration and loss of fluid.
  • Kidney function may be checked by measuring the BUN (blood urea nitrogen) and creatinine levels; this may be an important clue as well to the severity of dehydration.
  • Urinalysis may reveal dehydration if the specific gravity (urine concentration) is elevated or if there are ketones present.
  • Blood tests for markers of inflammation may also be measured, including erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). These are nonspecific tests that may help guide decision-making.
  • Stool samples may be collected for culture, searching for bacterial and parasitic infections as the cause of colitis. The stool may also be tested for blood.

What imaging tests and procedures diagnose colitis?


The length of the colon can be directly viewed by colonoscopy. A gastroenterologist uses a thin, flexible tube equipped with a fiberoptic camera to view the inside lining of the colon. The appearance of the colonic lining often allows the doctor to make the diagnosis and also provides the opportunity to look for tumors and polyps. Biopsies (small bits of tissue) can be obtained from the mucosal lining during colonoscopy and evaluated under the microscope by a pathologist to determine the cause of colitis. A biopsy is the only way to diagnose microscopic colitis.

Computerized tomography and barium enema

Computerized tomography and barium enema are tests that are sometimes ordered to help diagnose the potential cause of colitis. CT scan of the abdomen has become a more common test to evaluate patients with abdominal pain. However, the healthcare professional needs to balance the risk of radiation with the reward of the information that can be obtained. These tests usually are performed by a radiologist.

What is the treatment for colitis?

The treatment of colitis depends upon the cause.

  • For undiagnosed or uncontrolled colitis, the initial therapy (regardless of the cause) is to stabilize the patient's vital signs and help control pain if needed. Rehydration may be accomplished by mouth. However, for those patients who are markedly dry, who are unable to tolerate fluids by mouth, or who have electrolyte abnormalities, intravenous fluids may be required.
  • Medications are often used to control IBD and the choice of medication is tailored to the individual patient.
  • Antibiotics are not commonly used unless a specific bacteria is isolated and treatment is known to shorten the course of the infection.
  • Over-the-counter medications to treat diarrhea should be used with caution, especially if abdominal pain and fever are present. It is always wise to check with a care provider or pharmacist before taking antidiarrheal medications.
  • Surgery is usually not a treatment option for most causes of colitis.

What foods help soothe colitis flares?

Colitis often is associated with diarrhea, and the body can lose significant amounts of fluid with each episode of diarrhea. Moreover, the colon is inflamed, and it is important to try to "rest" it. Since clear fluids tend to be absorbed mostly in the stomach and small intestine, initially avoiding solid foods and promoting a clear fluid diet may be of help in rehydrating the body and resting the colon.

Changing your diet to reduce symptoms or flares may help soothe the symptoms of the disease, but there is no cure for some forms of colitis.

What foods should I avoid if I have colitis?

Depending on the cause, some people with colitis may find that certain foods bring on or make their symptoms worse. Keeping a food diary may be helpful and then avoiding foods that may be associated with worsening symptoms.

The Crohn's & Colitis Foundation of America recommends avoiding the following foods if you have colitis:

  • Greasy or fried foods
  • Milk or products containing milk
  • Certain high-fiber foods like popcorn, seeds, nuts, and corn

People who are lactose intolerant should avoid dairy products containing lactose. There are a variety of companies (for example, Lactaid and Green Valley Organics) that offer lactose-free products such as milk, yogurt, ice cream, and cottage cheese.

What is the prognosis for a person with colitis?

Patients with infectious diarrhea tend to get better relatively quickly with supportive care. Most infections will resolve with or without specific treatment and often do not require antibiotics. Those decisions depend on the patient's diagnosis.

Patients with inflammatory bowel disease probably will require lifelong treatment to help control their symptoms. The goal, as with any long-term illness, is to allow the patient to live a normal life with minimal symptoms of the disease.

Patients with ischemic colitis need to minimize their risk factors for progressive narrowing of the arteries. These are the same risks as heart disease and require the same treatment approach, including controlling the following:

Patients with severe ischemia that leads to a dead (gangrenous) colon require surgery to remove the gangrenous segment.

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Medically Reviewed on 7/11/2023
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Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.