Microscopic colitis is a mild chronic inflammation of the lining of the colon (large intestine). The most common symptom is nonbloody diarrhea. The exact cause is unknown but is sometimes associated with the use of certain medications. Microscopic colitis is diagnosed by taking tissue samples during a colonoscopy. Treatment involves stopping any potential offending medication, and the use of over the counter anti-diarrheal medication. Some patients may require steroids to control the inflammation and symptoms.