Investigators at the Imperial College of London (Journal of Psychopharmacology) quote on cannabis

"A direct comparison of alcohol and cannabis showed that alcohol was considered to be more than twice as harmful as cannabis to (individual) users and five times more harmful as cannabis to others (society). … As there are few areas of harm that each drug can produce where cannabis scores more (dangerous to health) than alcohol, we suggest that even if there were no legal impediment to cannabis use, it would be unlikely to be more harmful than alcohol."