Researchers asked drug-harm experts to rank these illegal and legal drugs on various measures of harm both to the user and to others in society. These measures include damage to health, drug dependency, economic costs and crime. The researchers claim that the rankings are stable because they are based on so many different measures and would require significant discoveries about these drugs to affect the rankings.
Note that alcohol (despite being legal and used more often than the other drugs) is by far the most harmful; not only is it the most damaging to societies, it is also the fourth most dangerous for the user. Most of the drugs were rated significantly less harmful than alcohol, with most of the harm befalling the user.

The authors explain that one of the limitation of this study is that drug harms are functions of their availability and legal status in the UK, and so other cultures' control systems could yield different rankings.
"Scoring drugs", The Economist, data from "Drug harms in the UK: a multi-criteria decision analysis", by David Nutt, Leslie King and Lawrence Phillips, on behalf of the Independent Scientific Committee on Drugs. The Lancet. 2010 Nov 6;376(9752):1558-65. doi:10.1016/S0140-6736(10)61462-6 PMID 21036393http://www.drugscience.org.uk/whatwedo/drugharms
User:Tesseract2