The action of antidepressants results in elevated levels of serotonin in the intestinal lining, which is associated with irritable bowel syndrome. Indeed, antidepressants have been found to cause the same symptoms as irritable bowel syndrome—pain, diarrhea, constipation, indigestion, bloating and headache. In a recent study, 14-23% of people taking antidepressants suffered these side effects.