Now that California’s billion-dollar " medical marijuana” industry and its affiliated “recommendationists” have made marijuana legally available to any Californian with $75 and the willingness to tell a doctor that he sometimes has trouble sleeping, why not go all the way and just legalize the stuff for recreational use as proposed in Proposition 19 on the November ballot? Then we could tax it and regulate it, eliminating the illicit market and the need for law enforcement against pot growers. California would make a ton of money to help dig out of its fiscal hole, right?

Well, actually, no.

There’s one problem with legalizing, taxing and regulating cannabis at the state level: It can’t be done. The federal Controlled Substances Act makes it a felony to grow or sell cannabis. California can repeal its own marijuana laws, leaving enforcement to the feds. But it can’t legalize a federal felony. Therefore, any grower or seller paying California taxes on marijuana sales or filing pot-related California regulatory paperwork would be confessing, in writing, to multiple federal crimes. And that won’t happen.

True, Atty. Gen. Eric H. Holder Jr. has announced that the Justice Department will not prosecute people who are selling medical marijuana in compliance with California’s law. But that’s an entirely different matter. The attorney general could cite good legal and constitutional reasons for that policy, because the regulation of medical practice is a state and not a federal responsibility. And if the medical justification for most of the pot sold through dispensaries is sketchy at best? Well, that too is a state problem. The international treaties that require their signatories, including the United States, to ban the production and sale of cannabis have an exception for medical use.

Most important, the feds can afford to take a laid-back attitude toward California’s medical marijuana trade because it’s unlikely to cause much of a trafficking problem in the rest of the country. Because dispensaries’ prices are just as high as those for black-market marijuana, there’s not much temptation to buy the “medical” sort in California and resell it out of state.

By contrast, the non-medical cannabis industry that would be allowed if Proposition 19 passed would quickly fuel a national illicit market. According to a study issued by the RAND Corp.'s Drug Policy Research Center this month, if the initiative passes, the pretax retail price of high-grade sinsemilla marijuana sold legally in California is likely to drop to under $40 per ounce, compared with current illicit-market (or dispensary) prices of $300 an ounce and more. Yes, the counties would have authority to tax the product, but even at a tax rate of $50 an ounce — more than 100% of the pretax price — the legal California product would still be a screaming bargain by national standards, at less than one-third of current black-market prices.

As a result, pot dealers nationwide — and from Canada, for that matter — would flock to California to stock up. There’s no way on earth the federal government is going to tolerate that. Instead, we’d see massive federal busts of California growers and retail dealers, no matter how legal their activity was under state law.

Even without the magnet effect of cheap drugs here, the feds couln’t afford to simply ignore a state’s flouting of the federal prohibition on marijuana. For one thing, allowing Californians to openly grow cannabis for non-medical purposes would be a clear violation of international law; that’s why the Netherlands, which tolerates retail cannabis sales through “coffee shops,” still bans marijuana production. As the Dutch say, the front door of the coffee-shop trade may be legal, but the back door is illegal.

So, then, should marijuana be kept strictly illicit throughout the United States? Not necessarily. Legalizing cannabis isn’t a terrible idea, but I’d very much prefer to do it on a non-commercial (grow-your-own or consumers’ co-op) basis rather than creating a multibillion-dollar industry full of profit-driven firms trying to encourage as much cannabis use as possible. The only way to sell a lot of pot is to create a lot of potheads — not casual, moderate recreational users but chronic, multiple-joints-per-day zonkers. (The alcohol industry, for example, gets 80% of its income from people with drinking problems.) A grow-your-own or co-op system would prevent that marketing push.

In any case, whenever and however we legalize the Demon Weed, it’s going to have to be at the national level (which includes modifying the anti-drug treaties) rather than state by state. Any other approach is a pipe dream.

Mark Kleiman is professor of public policy at UCLA and the editor of the Journal of Drug Policy Analysis. His latest book is “When Brute Force Fails: How to Have Less Crime and Less Punishment.”