What is the most dangerous drug in the world? This sounds like a relatively simple question: Surely it’s the one most likely to kill you, right? As it turns out, it depends on a multitude of things, from the individual owner's risk to the wider risk to society – and perception plays a large part.

How Do You Define “Dangerous?”

David Nutt is the Edmond J. Safra professor of neuropsychopharmacology at Imperial College London. He’s one of the world’s foremost experts on drugs, in terms of their use, their effects on the human brain, and international drug policy. Drug Science – formally the Independent Scientific Committee on Drugs – is a science-led drugs charity and research organization headed by Professor Nutt.

In 2010, a now-infamous paper was published by the group detailing their scientific analysis on the harms of drugs available in the U.K., both legal and illegal. Sixteen parameters of harm were chosen, and were divided in terms of the specific drug’s direct and individual effects on the user. A direct effect of a drug on a person could be death through an overdose, for example; an indirect effect could be damage caused by becoming infected with HIV while using contaminated syringes. Each drug’s effect on others and the wider society were also taken into account.

The list included mortality likelihood, dependence, impairment of mental functioning, loss of tangible socioeconomic things (such as a house or a job), physical injury, and criminal activities. The economic cost to the country, as well as the international damage (in terms of political and societal destabilization, for example) were also considered.

“Ranking twenty different drugs on sixteen different harms – that’s the best method we’ve had,” Professor Nutt told IFLScience. In a more general sense, the detrimental effects of drugs could be divided into two broad categories: harm to others and harm to users.

This group also had to weight the different criteria – some harms were considered, albeit subjectively, more important than others. “There are two elements to it,” Nutt continued. “Deciding on the various harms – the 16 parameters – most experts agree on that. The more interesting question is how much you care about each of these different rankings; this is where the weightings come in. This could vary greatly depending on the group’s opinions.”

A European group also attempted the same process a few years after the publication of this Lancet study, in collaboration with Drug Science. In terms of the two groups, the British prioritized economic harm more, whereas the Europeans ranked personal harm as considerably more important.