i watched a documentary about the different drugs/substances in our society, and ranked the drugs from most dangerous to least. They consulted psychologists, pharmacists etc and used other factors such as 'danger to society' as well as 'danger to physical health' etc. It was quite interesting, i think its on top documentaries .com (sorry for the ambiguous address, i watched it on my mates computer and didnt take note of how he got it)
anyway, from memory the results were
1. heroin
2. cocaine/crack
5. alcohol
~ 12. MDMA/Ecstasy
18. marijuana
Very interesting doco, only went for twenty min