In December 2013, Uruguay legalized marijuana, Earlier, in 2012, Colorado and Washington legalized marijuana under the laws of their states, and 21 additional states and the District of Columbia have now decriminalized or allowed medical use of marijuana. Portugal decriminalized all drugs in 2001, and the Netherlands has practiced de facto legalization for marijuana for decades. More broadly, many countries have de-escalated their “Wars on Drugs.” Indeed, President Obama hinted strongly in a recent interview that he supports marijuana legalization.

Legalization advocates, therefore, are feeling optimistic: Many expect full legalization, at least for marijuana, within a few years.

This euphoria is understandable, but premature.

Legalizers are correct that prohibition is a terrible approach to balancing the costs of drug abuse against the costs of policies that attempt to reduce drug abuse.

Prohibition drives drug markets underground, thereby generating violence and corruption. Participants in black markets cannot resolve their disputes with courts and lawyers, so they resort to violence instead.

Prohibition makes quality control difficult, so the incidence of accidental poisonings and overdoses is higher than in a legal market. People who purchase alcohol know what purity they are getting; people who purchase cocaine or heroin do not.

Prohibition spreads HIV. Elevated drug prices incentivize injection (users get a big bang for the buck), while fostering restrictions on clean needles. Users therefore share dirty needles, which accounts for a large fraction of new HIV infections in the United States.

Prohibition harms those who use drugs despite prohibition, since they risk arrest and imprisonment in addition to the negatives of drug use itself.

Prohibition encourages racial profiling and other infringements on civil liberties. Neither party to a drug transaction wants to notify the police, who therefore use more intrusive tactics in the attempt to enforce the law.

Prohibition wastes criminal justice resources and prevents collection of taxes on the production or purchase of drugs, thus adversely impacting government budgets.

And abundant evidence from America’s experiment with Prohibition, from state decriminalizations, and medicaliziations; from comparisons across countries with weak versus strong prohibition regimes; and from experience with other prohibited commodities suggests that prohibitions generates only moderate reductions in drug use. Some of that reduction, moreover, is a cost of prohibition, not a benefit—since many people consume drugs without ill effects on themselves or others.

Prohibition is therefore a terrible policy, even if one endorses government attempts to reduce drug use. Prohibition has large costs with minimal “benefits” at best in terms of lower use.

So legalizers are right on the merits, and recent opinion polls show increasing public supportfor legalization (at least for marijuana). But the negatives of prohibitions have been widely understood at least since the 1933 repeal of alcohol prohibition, yet this has not stopped the U.S. from pushing drug prohibition both at home and abroad.

In addition, further progress toward legalization faces serious impediments.

The first is that recent de-escalation of the Drug War addresses marijuana only. Yet much prohibition-induced harm results from prohibitions of cocaine, heroin, and methamphetamine. Public opinion is less open to legalizing these drugs.

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.