Cannabis potency and contamination: a review of the literature / Eurekalert, 17 June 2008
http://www.eurekalert.org/pub_releases/2008-06/w-clh061708.php
Jennifer McLaren, Wendy Swift, Paul Dillon, Steve Allsop
"Claims that a large increase in the strength of cannabis over the last decade is driving the occurrence of mental health and other problems for users are not borne out by a study of the worldwide literature, say researchers at the National Drug and Alcohol Research Centre (NDARC) and the National Drug Research Institute (NDRI), both from Australia."
http://www.blackwell-synergy.com/doi/full/10.1111/j.1360-0443.2008.02230.x