This was taken from The New England Journal of Medicine website.  The author of the article is Nora D. Volkow, M.D. and others.  

"Marijuana" as Neurotoxin...New England J. of Medicine

The galenical* melange of the herbal strains and chemical contents of cannabis sold and used as "marijuana," contains, among other things, a variety of long-acting psychoactive substances, the repeated consumption of which inevitably leads to a steady state within the brain, with unknown effects upon neurotransmission, receptor regulation, and cellular metabolism. Evidence is accumulating that the negative cognitive changes associated with continued use are irreversible. Acute and chronic affective changes are common knowledge. Evidence is accumulating that negative structural changes occur as well, within, at least, the limbic system, areas appropriately associated with motivation. Those who promote the industrialization of the horticulture of cannabis, and the development of "innovative" delivery systems (improved "cigarettes," confections, beverages, vaporizers) of the resultant neurotoxins, and those legislators who accept the taxes produced by the "legalization" of the agents, are, to my mind as a physician, neurologist, and neuroscientist, nothing more than the despicable purveyers of a slowly developing, generational neurotoxic catastrophe, which may be irreversible."

 *galenical: ‘a medicine prepared by extracting one or more active constituents of a plant’