(E)-2-[(2S,3S)-3-ethyl-8-methoxy-1,2,3,4,6,7,12,12b- octahydroindolo[3,2-h]quinolizin-2-yl]-3- methoxyprop-2-enoic acid methyl ester
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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In spite of the fact that mitragynine has sometimes been touted and used as a “legal opioid,” few scientific studies have addressed the psychoactive properties of mitragynine.  Most of the available information is based on anecdotal reports and patient experiences. The general subjective effects of mitragynine have been summarized in various reviews and include improved mood and analgesia, with some subjects experiencing relaxation and others stimulation (paradoxical effects).
Mitragynine itself acts primarily via μ-opioid receptors, though its oxidation product mitragynine pseudoindoxyl, acts as an even more potent and selective μ-opioid agonist but with less affinity for δ or κ receptors. Another alkaloid with a major contribution to the μ-opioid activity of the kratom plant is the related compound 7-hydroxymitragynine, which, while present in the plant in much smaller quantities than mitragynine, is a much more potent μ-opioid agonist. The extent to which this minor but more potent mu agonist constituent of the plant contributes to the subjective effects of Kratom consumption is still unclear. 
Mitragynine was isolated in 1907 by D. Hooper, a process repeated in 1921 by E. Field who gave the alkaloid its name. Its structure was first fully determined in 1964 by D. Zacharias, R. Rosenstein and E. Jeffrey.
Dry kratom leaf contains roughly 1-6% mitragynine. A typical dose ranges from 15 mg to 65 mg. A notable distinction between mitragynine and traditional opioids is that mitragynine does not cause hypoventilation (respiratory depression) and therefore does not carry the primary safety risk associated with traditional opioids.
Physically the freebase is a white, amorphous powder. It is soluble in alcohol, chloroform and acetic acid.
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