"In spite of the increasing use of this substance, scientific literature about the effects and toxicity of kratom alone remains very scarce.
"Kratom is a central nervous system stimulant, from which over 40 alkaloids have been isolated. In low doses it is reported to have stimulant effects (used to combat fatigue during long hours of work), while at high doses, it can have sedative-narcotic effects.87 In 1921, the major alkaloid found in this plant, ‘Mitragynine’, was first isolated. Mitragynine has an opioid agonistic activity and its derivative 7-hydroxymitragynine (7-OH-mitragynine) is reported to be more potent than mitragynine or morphine.88
"Nine fatal cases of intoxication associated with the use of ‘krypton’, a mixture of mitragynine and O-desmethyltramadol, have been described in scientific literature. However, these fatalities have been attributed to the addition of O-desmethyltramadol to the dried kratom leaves.89"


UN Office on Drugs and Crime, "The Challenge of New Psychoactive Substances: A Report from the Global SMART Programme" (Vienna, Austria: UNODC Laboratory and Scientific Section, March 2013), pp. 14-15.