Ulcerative colitis is a chronic disease in which abnormal reactions of the immune system cause inflammation and ulcers on the inner lining of your large intestine.

Ulcerative colitis can begin gradually and become worse over time. However, it can also start suddenly. Symptoms can range from mild to severe. In between periods of flares—times when people have symptoms—most people have periods of remission—times when symptoms disappear. Periods of remission can last for weeks or years. The goal of treatment is to keep people in remission long term.

Ulcerative colitis is an inflammatory bowel disease (IBD). Research suggests that about 600,000 to 900,000 people in the United States have ulcerative colitis.

Symptoms of ulcerative colitis vary from person to person. Common symptoms of ulcerative colitis include:

• diarrhea
• passing blood with your stool or rectal bleeding
• cramping and pain in the abdomen
• passing mucus or pus with your stool
• tenesmus, which means feeling a constant urge to have a bowel movement even though your bowel may be empty
• an urgent need to have a bowel movement

Symptoms of ulcerative colitis may vary in severity. For example, mild symptoms may include having fewer than four bowel movements a day and sometimes passing blood with stool. Severe symptoms may include having more than six bowel movements a day and passing blood with stool most of the time. In extremely severe—or fulminant—ulcerative colitis, you may have more than 10 bloody bowel movements in a day.

Some symptoms are more likely to occur if ulcerative colitis is more severe or affects more of the large intestine. These symptoms include:

• fatigue, or feeling tired
• fever
• nausea or vomiting
• weight loss

Ulcerative colitis is more likely to develop in people:

• between the ages of 15 and 30, although the disease may develop in people of any age
• who have a first-degree relative—a parent, sibling, or child—with IBD
• of Jewish descent

Doctors aren’t sure what causes ulcerative colitis. Experts think that the following factors may play a role in causing ulcerative colitis:

Genes

Ulcerative colitis sometimes runs in families. Research suggests that certain genes increase the chance that a person will develop ulcerative colitis.

Abnormal immune reactions

Abnormal reactions of the immune system may play a role in causing ulcerative colitis. Abnormal immune reactions lead to inflammation in the large intestine.

Microbiome

The microbes in your digestive tract—including bacteria, viruses, and fungi—that help with digestion are called the microbiome. Studies have found differences between the microbiomes of people who have IBD and those who don’t. Researchers are still studying the relationship between the microbiome and IBD.

Environment

Experts think a person’s environment—one’s surroundings and factors outside the body—may play a role in causing ulcerative colitis. Researchers are still studying how people’s environments interact with genes, the immune system, and the microbiome to affect the chance of developing ulcerative colitis.

Ulcerative colitis may lead to complications that develop over time, such as:

• anemia, a condition in which you have fewer red blood cells than normal. Ulcerative colitis may lead to more than one type of anemia.
• bone problems, because ulcerative colitis and corticosteroids used to treat the disease can affect the bones. Bone problems include low bone mass, such as osteopenia or osteoporosis.
• problems with growth and development in children, such as gaining less weight than normal, slowed growth, short stature, or delayed puberty.
• colorectal cancer, because patients with long-standing ulcerative colitis that involves a third or more of the colon are at increased risk and require closer screening.

In some cases, ulcerative colitis may lead to serious complications that develop quickly and can be life-threatening. These complications require treatment at a hospital or emergency surgery. Serious complications include:

• fulminant ulcerative colitis, which causes extremely severe symptoms, such as more than 10 bloody bowel movements in a day, often with fever, rapid heart rate, and severe anemia. People with fulminant ulcerative colitis have a higher chance of developing other complications, such as toxic megacolon and perforation.
• perforation, or a hole in the wall of the large intestine.
• severe rectal bleeding, or passing a lot of blood from the rectum. In some cases, people with ulcerative colitis may have severe or heavy rectal bleeding that may require emergency surgery.
• toxic megacolon, which occurs when inflammation spreads to the deep tissue layers of the large intestine, and the large intestine swells and stops working.

Severe ulcerative colitis or serious complications may lead to additional problems, such as severe anemia and dehydration. These problems may require treatment at a hospital with blood transfusions or intravenous (IV) fluids and electrolytes.

Ulcerative colitis increases the chance of getting colorectal cancer. People have a higher risk for developing colorectal cancer if ulcerative colitis affects more of their large intestine, is more severe, started at a younger age, or has been present for a longer time. People with ulcerative colitis also have a higher risk of developing colorectal cancer if they have primary sclerosing cholangitis or have a family history of colorectal cancer.