OPRM1 and respiratory depression: Unlike classical opioids, which act as full agonists at mu opioid receptors, kratom’s two primary and best understood bioactive alkaloids, mitragynine and 7-hydroxymitragynine, act at mu opioid receptors as partial putatively “biased” agonists, meaning that they do not contribute to significant respiratory depression in pre-clinical animal studies (as discussed more below), making “poisoning”, when kratom alone is used, a highly questionable cause of death.