It dosent matter if its synthetic as long as there no impurities in the substance. Synthetic mitragynine would bw the exact compound as the plant derived extract.

Just because it comes from a plant does not mean its good for you anyway.
Strychnine is plant derived.

Aconitine is derived from aconitium napellus appearing in mythology when Arachne challenged the Greek goddess Minerva to a weaving contest. after Arachne wove her web and hung herself Minerva preserved Arachnes memory "Then, sprinkling her [Arachne] with the juices of aconite, the goddess [Minerva] transformed her into a spider. (Gayley, C.M., The Classic Myths in English Literature and Art, Ginn-Blaisdell Xerox company, 1893, 1911, and 1939. Massachusetts, Toronto, London. pgs. 82-84)

Or the well known 'suicide tree' in southern India has been responsible for approximately 50 deaths a year in Kerala alone not to mention Madagascar were it has estimatedly killed 3000 people per year. It is the plant cerbera odollam containing a extremely deadly poison 'cerberin' causing the heart to stop beating after 3 - 6 hours. It has only recently been investigated by our weastern toxicologist escaping detection by forensics until recently. (Gaillard, Y., Krishnomoorthy, A., Bevalot, F., Journal of Ethnopharmacology 95 (2004) 123-126)