OPRM1 and allergic disease: Loperamide is a mu-opiate-receptor agonist that extends the orocecal and colonic transit times by increasing gut activity, disarranging electrical activity in the gut and delaying the passage of fluid through the small intestine, although it has some side effects such as severe allergic reaction, toxic megacolon, constipation, decreased urination and stomach bloating [31,32].