Diarrhea after drinking coffee may be caused by coffee stimulation or diseases such as irritable bowel syndrome and colitis. 1. Coffee stimulation: coffee contains caffeine and other substances, if you drink coffee on an empty stomach, it may stimulate the gastrointestinal tract peristalsis, which may lead to diarrhea after drinking coffee. 2. Irritable bowel syndrome: Irritable bowel syndrome is a common dysfunctional syndrome, in which patients experience abdominal pain, bloating, and changes in bowel habits. The cause of irritable bowel syndrome is not yet clear, but studies have shown that dietary factors can aggravate or trigger the symptoms of patients, such as diarrhea when drinking coffee. 3. Colitis: Colitis is an inflammatory disease of the colon caused by environmental, genetic, microbial, and immune factors, with the main symptoms being abdominal pain, diarrhea, or constipation. Improper diet can aggravate the symptoms, which can manifest as diarrhea when drinking coffee. There are many causes of diarrhea after drinking coffee, and if necessary, medical examination should be conducted to clarify the cause.