Jon Caulkins has a first-rate essay in National Affairs on what the numbers actually say about the dangerousness of cannabis.
The real trouble is not that marijuana is more or less dangerous than alcohol; the problem is that they are altogether different, and the comparison is simply unhelpful in informing the debate over marijuana policy. The country is not considering whether to switch the legal statuses of alcohol and marijuana. Unfortunately, our society does not get to choose either to have alcohol’s dangers or to have marijuana’s dangers. Rather, it gets to have alcohol’s dangers — modulated perhaps by higher or lower drinking ages and higher or lower taxes — and also marijuana’s dangers — modulated by how legalization or prohibition affect prices, product variety, marketing, and usage.
Instead of comparing the harms of marijuana when it is prohibited to the harms of alcohol when it is legal, an intellectually honest marijuana-policy analysis ought to compare all possible harms under marijuana prohibition to all possible harms under legalization. And that analysis ought also to worry about indirect effects on abuse of other illicit drugs, alcohol, and tobacco — outcomes for which the current evidence is scant, divided, and discouraging, respectively.