https://drive.google.com/file/d/0BwMPT92bOJKdZWM2bzlLeU5DdDg... is the letter from the American Kratom Association to the DEA, which compares the number of kratom related calls to poison control centers to calls related to supplements. Kratom seems to be at about the same level as ginkgo biloba and better tolerated than St. John's Wort.
This is surprising, and has resulted in weird errors in literature reviews on kratom, that I've spent dozens of hours trying to unpack during my efforts to improve the Wikipedia article.
Kratom seems to have a really surprisingly low incidence of severe side effects. The state of the information out there is weird. Something like someone said "Kratom is a mu opioid agonist and mu opioid agonists cause respiratory depression" in a peer reviewed paper, which then got cited in a medical textbook as "Kratom causes respiratory depression". However, primary research in mice shows that kratom causes dramatically less respiratory depression than poppy derived opioids. No case of kratom related respiratory depression has actually been documented in a human. In my comprehensive review of the literature, there was someone who had a seizure and was intubated but still had respiration in the normal range. Someone else suffered aspiration pneumonia in response to (inhaling?) kratom, but aspiration pneumonia is not respiratory depression.
Basically, kratom does have side effects, and real side effects, but when looked at as a population level public health problem, is profoundly safe for how powerful of an effect it actually has.