However it is true that drugs like heroin (when pure) are quite safe overall - aside from the respiratory depresssion overdose scenario.
I haven't checked their sources, though.
In general taking excessive amounts triggers vomiting, similar to when you eat too many psilocybin mushrooms (well, some say it’s ideal to vomit shroom-wise, but I’m not a fan). So mitragyna speciosa is incredibly safe.
Unfortunately Wikipedia’s treatment of the topic could be improved, but there is very little existing literature out there. I read one review study I was pretty happy with but I’m not on a laptop so i can’t go dig up the link unfortunately
They spend billions on developing and promoting these, and they can't figure out the side effects for a specific drug? Maybe they should spend just a little bit more.