Even worse, drug warriors might respond to marijuana legalization by ramping up hysteria toward still-prohibited drugs, increasing prohibition-induced ills in those markets. The public would then observe increased drug-market violence in the wake of marijuana legalization, which would appear to show that legalization causes violence.

A different worry is that while public opinion currently swings toward legalizations, public opinion can change. And marijuana remains illegal under federal law, so a new president could undo President Obama’s “hands off” approach.

Perhaps the greatest threat to legalization is that many people—including some legalizers—believe policy can eliminate the black market and its negatives while maintaining strict control over legalized drugs. That is why recent legalizations include restrictions on production and purchase amounts, retail locations, exports, sales to tourists, high taxes, and more.

If these restrictions are so weak that they rarely constrain the legal market, they do little harm. But if these restrictions are serious, they re-create black markets.

Legalizers must accept that, under legalization, drug use will be more open and some people will misuse. The incidence of use and abuse might be no higher than now; indeed, outcomes like accidental overdoses should decline. But legalizers should not oversell, since that risks a backlash when negative outcomes occur.

None of this is meant to deny that recent policy changes constitute real progress. But these gains will evaporate unless the case for legalization includes all drugs and is up front about the negatives as well as the positives.

Jeffrey A. Miron is Senior Lecturer and Director of Undergraduate Studies in the Department of Economics and a Senior Fellow at the Cato Institute. He is the author of “Libertarianism, from A to Z.”