The April issue of the scientific journal Alcoholism: Clinical and Experimental Results is scheduled to contain a report analyzing brain scans of teenage alcohol and marijuana users. The results indicate alcohol is highly destructive to the brain's white matter- but the scans show no negative consequence from cannabis use.
The scans were performed at the beginning of the study and again at the end, 18 months later. Extensive toxicology reports were performed every six months on the study's 92 adolescent participants. The study evaluated the brain scans only, not intelligence or behavior.
Dr. Tappert, lead researcher on the project, cited the difficulties of studying marijuana on human test subjects. "One reason is that marijuana can really vary. Different strains contain different levels of THC and other marijuana components. For example, some studies have suggested one component, cannabidiol, may actually have neuroprotective effects."
More details on the upcoming report can be found in an excellent article on Huffington Post. That article contains references to other studies and potential conclusions from the as-yet unpublished data.
Source: The Compassion Chronicles