Activation of MOR, KOR and DOR leads to strong analgesia, but also to subtype specific side effects.4 Most of the clinically used analgesics are MOR agonists, such as morphine and methadone, which cause euphoria, constipation, respiratory depression, development of tolerance and addiction side effects.5 In contrast, KOR agonists do not elicit these typical MOR‐mediated side effects. This evidence concerns the gene OPRM1 and constipation disorder.