DDC and irritable bowel syndrome: Peripheral serotonin is an important regulator and is implicated in several diseases, such as osteoporosis,64 diarrhea-predominant irritable bowel syndrome,65 celiac disease,66 inflammatory bowel disease,67 and obesity.68 Marketed drugs: Carbidopa and benserazide, developed as human AADC inhibitor, significantly inhibited PEA production in En.