Scientists created a scale of drug harm that rates the damage to drug users themselves and to wider society.
Alcohol was rated the most harmful drug overall, and it was rated almost three times as harmful as cocaine or tobacco.
Heroin and crack cocaine rank as the second and third most harmful drugs.
The research appears in the Lancet medical journal.
The World Health Organization estimates that risks linked to alcohol are responsible for 2.5 million deaths a year from heart and liver disease, road accidents, suicides and cancer.