Running head: ranking prescription opioidsKey words: opioids, MCDA, risk assessment, ranking, prescription drugs, harm,street heroin Both forms of heroin scored very high: overall harm score of 99 for injected heroin and 72 for smoked heroin on a scale of 0-100. The main feature that distinguishes both forms of street heroin use is that their harm to others is more than five times that of the other eleven opioids. The overall harm score of fentanyl (including injection of fentanyl extracted from patches) and diamorphine (medically prescribed form of heroin) was 54 and 51, respectively, whereas that of orally used opioids ranged from 32 (pethidine) to 11 (codeine-containing pharmaceuticals).Injected street heroin, fentanyl and diamorphine emerged as most harmful to users, with the latter two very low in harm to others.Pethidine, methadone, morphine and oxycodone are also low in harm to others, while moderate in harm to users.We conclude that the overall harms of non-medically used prescription opioids are less than half that of injected street heroin.These data may give a basis for precautionary regulatory measures that should be considered if the rising trend in non-medical use of prescription opioids were to become evident in the UK.