Why does ulcerative colitis cause diarrhoea?

The large bowel normally removes water from stool before it leaves the body. Ulcerative colitis inflames its inner lining, which can interfere with this job and make the bowel more sensitive to stool and gas. The result may be loose stools, urgency, mucus and a feeling that you need to pass stool again soon after going. Diarrhoea is a common presentation of Ulcerative Colitis, but diarrhoea alone does not identify the cause.

Symptoms may become more active during a flare and can also be affected by infection, food intolerance or a medicine side effect. A change from your usual bowel pattern is useful information for your care team.

Drink regularly, using oral rehydration solution if a clinician or pharmacist recommends it, because loose stools can deplete fluid and salts. Eat simple foods in small portions if they are easier to manage. Continue your prescribed ulcerative colitis medicine unless you are told otherwise, and contact your clinician to report a new or persistent change in bowel habits.

Write down stool frequency, urgency, blood or mucus, abdominal pain, fever and recent medicine changes. This record can help distinguish a possible flare from another cause and guide the next step.

Get urgent medical advice if you cannot keep fluids down, are passing very little urine, feel faint or confused, have severe abdominal pain, develop a swollen abdomen, or pass black stools or substantial blood. Seek prompt assessment for fever with worsening diarrhoea, because infection may need different treatment from an inflammatory flare.

Ask whether the diarrhoea could reflect active ulcerative colitis, infection or a treatment effect. Ask what fluid replacement is suitable, whether tests are needed, and when to contact the service again if symptoms do not improve.