Toxic Megacolon: UC Emergency Warning Signs (Plain English)
Last Updated Jan 15, 2026
Toxic megacolon is a rare but very serious complication that can happen during severe inflammation or infection in the colon, including during a severe ulcerative colitis (UC) flare. In plain English, it means the large intestine swells and stops moving normally, so gas and stool can build up and the colon can become dangerously enlarged. This is treated as a medical emergency because the colon can tear and serious whole-body infection can occur. [1]
Early warning signs that need urgent evaluation
Many toxic megacolon symptoms in ulcerative colitis can look like “a really bad flare” at first, so the pattern matters. A key clue is abdominal distension (the belly looks or feels more swollen than usual), especially when it shows up along with signs that the whole body is under stress. In UC, this kind of swelling is not just uncomfortable, it can signal that the colon is becoming enlarged and less able to push gas through.
Common toxic megacolon signs described by patient organizations include belly pain, abdominal swelling or distension, fever, and a rapid heart rate. Some people also have constipation (or suddenly pass less stool and gas than expected), plus dehydration. [2]
Because this topic often comes up during flares and emergency room decisions, many care plans treat the combination below as a “do not wait” situation for urgent evaluation:
- Worsening belly swelling with increasing pain or tenderness
- Fever and fast heartbeat (fever and tachycardia in UC can be a sign of severe inflammation)
- New confusion, dizziness, faintness, or very low energy
- Signs of dehydration, especially if fluids cannot be kept down
These symptoms do not confirm toxic megacolon on their own, but they are the kind of severe UC complications that clinicians generally want assessed quickly in an emergency setting.
Who is at higher risk, and what urgent evaluation typically involves
Toxic megacolon is usually linked to severe colitis (inflammation of the colon) and is more likely when the body is already stressed by a strong inflammatory flare or a colon infection. Some known risk factors during severe ulcerative colitis include electrolyte problems (such as low potassium or low magnesium), bowel preparation (the “cleanout” done before certain procedures), and the use of anti-diarrheal medicines. [3]
In the emergency room, evaluation typically focuses on two goals: confirming whether the colon is dangerously dilated, and checking for whole-body toxicity. Clinicians often check vital signs (temperature, heart rate, blood pressure), do a careful abdominal exam, and order tests that look for inflammation, anemia, dehydration, and infection. Imaging is commonly used to look for colon dilation and to help rule out perforation. Toxic megacolon is often described using a mix of colon dilation plus systemic warning signs such as fever, fast heart rate, dehydration, low blood pressure, or mental confusion. [4]
If a person is hospitalized for a severe UC flare, gastroenterology guidelines emphasize close monitoring (stool frequency, rectal bleeding, physical exam, vital signs, inflammatory markers), assessing for toxic megacolon, and involving surgical specialists early when there is suspected toxicity or poor response to initial treatment. The same guidance also notes that certain pain medicines and other drugs that slow the bowel are generally avoided in acute severe ulcerative colitis. [5]