A new working paper from the National Bureau of Economic Research contains some inconvenient news for the rosy worldview of those who claim that marijuana is a completely harmless drug.

The paper reviews data on opioid and marijuana use and makes two key findings — first, that “medical marijuana, particularly when available through retail dispensaries, is associated with higher opioid mortality.” The second finding is that data “for recreational marijuana, while less reliable, also suggest that retail sales through dispensaries are associated with greater death rates relative to the counterfactual of no legal cannabis.”

The increase in opioid deaths associated with marijuana use is greater for men, nonwhites, and young people.

That may be somewhat disturbing, but the details appear even more devilish. The study importantly addresses earlier results, based on data from 1999 to 2010, that had seemed to suggest a more beneficial effect. It turns out, though, that the results abruptly changed. If you include data from 2010 to 2017, the period when medical and/or recreational marijuana legalization began in earnest in the states, the results swing from a 21% reduction in opioid deaths to a 23% increase.

The results are complex, but the study undercuts a key claim of marijuana legalization proponents who argue that marijuana is a harmless substance that causes a cheap, temporary high and nothing more.

Here, the relationship between cannabis and opioid deaths is interesting in that it reinforces a much-mocked description of marijuana as a “gateway drug.” Different drug habits might well be related in ways we do not yet understand.

And of course, this is not the only pitfall associated with marijuana use that marijuana campaigners work hard to minimize. For example, habitual marijuana use as late as one’s mid-20s can cause permanent brain damage. That’s because it prevents proper development of the frontal cortex, which the American Psychological Association describes as one of the last regions of the brain to develop fully. This brain structure is “critical to planning, judgment, decision-making, and personality.”

This means that use among teenagers, which is a lot more common than people would like to admit, and even use among young adults has deleterious and permanent health effects.

This is something to remember when these campaigners come to your state and try to sell you on the idea of cannabis as something completely harmless. One need not exaggerate the dangers of marijuana to acknowledge that they at least exist.