According to the International Foundation for Functional Gastrointestinal Disorders, drinks containing caffeine cause the body to react much like a laxative would; however, exceeding two to three cups of coffee or tea daily can trigger diarrhea.
Peristalsis, the contraction and relaxation of intestinal muscles, is stimulated by the caffeine in coffee. This causes bowels to move at quicker rate. Stools may also be looser because the colon does not have time to reabsorb waters as it normally would.
Ingesting large amounts of caffeine has been known to cause diarrhea or worsen the symptoms of Irritable Bowel Syndrome. Eliminating or replacing caffeinated beverages with decaf options often helps alleviate these problems.