United States Attorney-General Jeff Sessions is pushing the Federal Government back into cannabis enforcement. This is an unwise and unnecessary move that may divert resources from more serious problems, and end up backfiring on those who want to restrain pot use.

Sessions rescinded on Thursday a policy that kept the Federal Government largely out of the way of states that have legalised cannabis. A majority of states have now legalised it in some form. Maryland just began permitting medical cannabis. California just legalised recreational cannabis, and Vermont is near to doing so.

Sessions’s move upended a tenuous deal the Obama Administration made with legalisation states: keep pot out of minors’ hands and help to combat trafficking, and federal authorities will focus on bigger priorities. This policy allowed a handful of states room to experiment with unencumbered legalisation, which would have made the consequences clearer to others.

The decision by Sessions is unlikely to result in arrests of small-time cannabis users. But it will chill the growth of the above-board weed economy by deterring banks and other institutions from participating. From there, US Attorneys across the country will decide whether to crack down, and on whom — a few big distributors, perhaps, or a few local grow shops, too. In states with complex regulations on cannabis growing, testing and selling, some operations may move back underground rather than provide documentation to state authorities that federal prosecutors may later use against them.

Sessions’s move is counterproductive even for sceptics of legalisation, whose only defence against a growing tide of public opinion would be evidence that full legalisation has significant negative consequences. His actions diminish the possibility of drawing lessons, including cautionary ones, from the examples of legalisation states. Similarly, Sessions has made it harder to learn how to regulate the legitimate weed economy, if that is the path the country chooses.

More concerning is the prospect that US Attorneys will begin diverting limited federal resources into anti-pot campaigns from far more pressing matters. As Sessions himself said this past November, the nation is experiencing “the deadliest drug crisis in American history”. That would be the opioid epidemic, which, Sessions noted, claimed some 64,000 lives in 2016. Cannabis simply does not pose the same threat, and the Attorney-General should have avoided any suggestion that it requires more attention right now.

Sessions’s decision will spur calls for Congress to finally change federal law. That is warranted, but lawmakers should be wary of swinging too far in the opposite direction. As a recent National Academies of Science review found, experts still know relatively little about the health effects of cannabis.

It makes no sense to lock up small-time cannabis users, but it may not make sense to move quickly to national legalisation. Rather, Congress should decriminalise cannabis use, then await more information.