Politifact: Marijuana is 100 times less toxic than alcohol.
You can read more here at the article. I believe they got a lot of information from the Marijuana Policy Project which is a great organization.
You can read the full article here:
There is an article on Huffington Post where Mason Tvert explains that "It is one thing for our federal officials to convey their opposition to marijuana policy reform. It is an entirely different and more disturbing situation when they are conveying opposition to scientific evidence."