Nutt and colleagues' 'rational' scale to assess the harms of commonly used drugs was based on ratings by a panel of experts. This survey aimed to assess drug users' views of the harms of drugs using the same scale. As users' drug choices are not solely based on harms, we additionally assessed perceived benefits. The survey was hosted at http://www.nationaldrugsurvey.org. UK residents reported their experience of 20 commonly used substances; those with direct experience of a substance rated its physical, dependence-related and social harms as well as benefits. A total of 1501 users completed the survey. There was no correlation between the classification of the 20 drugs under the Misuse of Drugs Act and ranking of harms by users. Despite being unclassified substances, alcohol, solvents and tobacco were rated within the top ten most harmful drugs. There was a remarkably high correlation (r = 0.896) overall between rankings by users' and by experts. Ecstasy, cannabis and LSD were ranked highest by users on both acute and chronic benefits. These findings imply that users are relatively well informed about the harms associated with the drugs they use. They also suggest that the current UK legal classification system is not acting to inform users of the harms of psychoactive substances.