While this argument may indeed seem a sensible prioritizing of federal effort and dollars, the White House and the public should realize it comes with a cost.

That cost is Washington's tacit approval of state-sanctioned medical marijuana, which the drug's proponents will take as a green light to push even harder for their ultimate goal: full legalization of marijuana use and distribution.

Backers would like to see the buying and selling of pot regulated and taxed much like alcohol and tobacco. Their patient and well-funded route to this goal is through the states, with one avenue being state legalization of medical marijuana.

Since 1996, 13 states have allowed such use - in defiance of a federal statute that outlaws marijuana as a controlled substance. Meanwhile, the Food and Drug Administration does not approve of marijuana as safe or effective for any medicinal use, and the drug has never gone through the FDA's rigorous approval process. Yet several more states are considering medical marijuana laws at the urging of pro-pot and patients-rights groups.

Their stated reason is compassion. They argue that marijuana alleviates suffering for certain illnesses. No one wants to deny compassion for the sick, but Americans need to be aware of the larger context in this debate.

To begin with, state medical marijuana laws have opened the door to distribution beyond those who are ill. California shines glaringly as Exhibit A - just like the green neon signs that advertise so many of its medical marijuana dispensaries.

The Golden State was the first to legalize marijuana for medical purposes and its law is notoriously loose. All it takes to buy pot is a doctor's permission - and some doctors are willing to fill prescriptions on the thinnest of pretenses. With hundreds of marijuana storefronts in Los Angeles, the city put a moratorium on new dispensaries in 2007. A Superior Court judge ruled this week that an extension of the moratorium is invalid, a move that is likely to spur the opening of even more pot shops.

The federal regulation question gets at another fundamental issue - the safe use of marijuana as a medicine. The FDA is not alone in its refusal to sanctify marijuana for medical purposes. Neither does the American Medical Association approve of it - though it has encouraged its study. And questions linger about dosage, purity, and unpredictability.

Generally, marijuana is not nearly as harmless as its proponents make it out to be. While pot cannot directly kill the way that alcohol or, say, an overdose of heroin can, heavy use can lead to dependence. About 1 in 10 people who have used marijuana become dependent at some time, according to Kevin Sabet, in the 2006 book, "Pot Politics." Mr. Sabet, a staunch opponent of legalizing marijuana, is now a policy adviser to the president's drug czar, Gil Kerlikowske.

Heavy use can also lead to serious mental-health problems, especially in young people. Even casual use distorts perception, reduces motor skills, and affects alertness - a hazard in driving and other activities.

Thankfully, the Obama administration does not support the legalization of marijuana. And this week's Justice Department directive, which formalized a decision taken last March, by no means lets dispensaries off the hook. The feds will still go after misuse of state medical marijuana laws - prosecuting providers that serve minors, launder money, or illegally possess firearms.

"We will not tolerate drug traffickers who hide behind claims of compliance with state law to mask activities that are clearly illegal," Attorney General Eric Holder said.

All well and good. The problem is, the line between legal and illegal regarding marijuana is fading year by year. The pro-pot groups would rub it out altogether. For the sake of a clear-thinking and healthy America, that must not be allowed to happen.