There are several different ways of measuring how much harm drugs can cause, according to British psychiatrist David Nutt.
You could take into account how much harm a drug is responsible for in society, how addictive it is, or just how dangerous it is for an individual user.
For a 2007 study published in The Lancet, Nutt convened two groups of independent experts to rate the addictiveness and harmfulness of 20 different drugs (the ratings of one group of psychiatrists were used to corroborate the assessment of a larger group).
These are the drugs that Nutt and colleagues rated as most dangerous to the people using them:
Heroin, also the most addictive drug, scores highest because of the immediate risks it can pose to users, the long-term risks associated with chronic use, and the high risk that's added by intravenous use.
The intravenous rating is based on the likelihood of intravenous use. That's a big factor since that method of administration has a high risk of causing sudden death and is associated with spreading chronic disease as well.
Other drugs that fall higher on scales of addictiveness or general harm don't rate quite as highly on the individual danger scale.
For example, tobacco, which scores higher than any other drug for the risks associated with chronic use (2.9 out of 3), doesn't make the top 10 here because of a much lower score for immediate acute risk and a zero score for intravenous use.
Alcohol also doesn't make the top 10 for risks to the individual user, but actually is the drug that Nutt and colleagues rate as causing the most overall harm, once you include addictiveness and especially once you count the impact that the use of alcohol has on others, due to the effects of intoxication, social harm, and related healthcare costs.
You can check out Nutt's full analysis of the harm and addictiveness ratings of drugs in Table 3 of the study; he calculated the mean score of each of the three variables (acute harm, chronic risks, and intravenous factors) rather than adding them up as we did here.
