"In Southeast Asia, kratom has long been used for the management of pain and opium withdrawal.6,9-11,14 In the West, kratom is increasingly being used by individuals for the self-management of pain or withdrawal from opioid drugs such as heroin and prescription pain relievers.20,27 It is these aspects of kratom pharmacology that have received the most scientific attention. Although to our knowledge, no well-controlled clinical studies on the effects of kratom on humans have been published, there is evidence30-38 that kratom, kratom extracts, and molecules isolated from kratom can alleviate various forms of pain in animal models. Studies have used a variety of methods including hot plate,35,37,39 tail flick,32,39 writhing,37,38 and pressure/inflammation35,38 tests in mice32,35,38,39 and rats,35,37 as well as more elaborate tests in dogs and cats.35 In addition, a variety of chemical compounds have been isolated from kratom and shown to exhibit opioid-like activity on smooth muscle systems31,33,34 and in ligand-binding studies.39,40 Most notably, many of the central nervous system and peripheral effects of these kratom-derived substances are sensitive to inhibition by opioid antagonists.31-34,39-41"
Prozialeck, WC, Jivan, JK, and Andurkar, SV. Pharmacology of kratom: an emerging botanical agent with stimulant, analgesic and opioid-like effects. Journal of the American Osteopathic Association. December 2012;112(12):792-9.