I hadn’t known that opioids generally caused vomiting - I’ve only needed codeine in fairly small quantities on a few occasions, so little chance to learn that.
I was mistaken about the receptors - it seems kratom interacts with all the opioid receptors.
http://www.emcdda.europa.eu/publications/drug-profiles/kratom notes “The withdrawal symptoms in humans are relatively mild and typically diminish within a week.”
https://en.wikipedia.org/wiki/Mitragyna_speciosa#Respiratory_depression mentions that e.g. “In animal studies at very high doses, mitragynine caused respiratory depression, but less than morphine or codeine.” and that the CDC’s 2016 report on Kratom exposures lists symptoms observed - nausea, drowsiness, tachycardia etc. - but not respiratory depression
