Not very convincing sources but believe what you will. There are sources to support anything anyone wants to believe these days.
There will be no proof strong enough ever for those who want their recreational drugs. Facts need not be posted.
http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=19709
http://www.addiction-help-line.com/marijuana.html
http://www.drug-rehabilitation.com/marijuana-addiction/
http://www.pslgroup.com/dg/83B4A.htm
http://health.yahoo.com/health/centers/addiction/96407687