Alcohol is more harmful than heroin or crack, according to a study published in the Lancet medical journal.
The report is co-authored by Professor David Nutt, a former British chief drugs adviser.
It ranks 20 drugs on 16 measures of harm to users and to wider society.
Tobacco and cocaine are judged to be equally harmful, while ecstasy and LSD are among the least damaging.
Professor Nutt was sacked from his post by former Labour Home Secretary Alan Johnson in October 2009.
The BBC reports he went on to form the Independent Scientific Committee on Drugs, a body which aims to investigate the drug problem without any political interference.