Unexpected potholes are popping up on the road to legal pot. Amid polls showing a majority of North Americans favouring legalization, new voices of caution are challenging the views of the weed liberationists who would tax and regulate cannabis like any other state-sanctioned high.

These apparent killjoys are not the usual suspects tsk-tsking in disapprobation.

No one is surprised to see the federal Conservatives seize on Justin Trudeau's support for legalization as a wedge issue to mobilize their base and cast doubt on the Liberal Leader's moral probity. It's more surprising to hear prominent yuppie voices express doubt about the wisdom of legalizing recreational pot.

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Former New Yorker, Newsweek and Daily Beast editor Tina Brown tweeted her disapproval this week, saying "legal weed contributes to us being fatter, dumber, sleepier nation even less able to compete with the Chinese."

New York Times columnist David Brooks equated Colorado's pot legalization with America's moral decay, suggesting that "in healthy societies … government subtly encourages the highest pleasures, like the arts or enjoying nature, and discourages lesser pleasures, like being stoned."

My own concerns about legalizing pot aren't moral; they're practical. The scientific jury is out on the effects of regular marijuana use. U.S. President Barack Obama, who admitted puffing pot prodigiously in high school, turned out okay. Rob Ford "smoked a lot of it," with the results we've all come to know. Pot is surely no more dangerous than alcohol and, legal or not, healthy adults need to self-regulate.

But anyone who has seriously studied the issue – it's not clear Mr. Trudeau qualifies – has concluded that legalizing marijuana is a lot harder than it sounds. It raises at least as many policy questions as it purports to solve. Those in favour need to provide some answers.

First off, we need to make the distinction between legalization and decriminalization. Even Prime Minister Stephen Harper's Conservatives are warming to the idea that it makes no sense to clog up our overburdened justice system with minor charges for pot possession. The Canadian Association of Chiefs of Police favours the option of issuing ticketed fines to those caught with an ounce or less.

That seems like a logical first step, one that's long overdue. As prime minister, Mr. Trudeau's father, Pierre, repeatedly promised to decriminalize marijuana. And when he dragged his feet, the Progressive Conservatives were the first to criticize him for it. But decriminalization efforts died in 1980s, as U.S. first lady Nancy Reagan admonished young people to "Just Say No" and her husband, Ronald, called marijuana "probably the most dangerous drug in America."

By the time I got to university during Mr. Reagan's second term, pot seemed passé, at least outside the drama and art history departments. But the lull was temporary. Marijuana re-emerged as the hipster's drug of choice and is now as prevalent as acne in North American high schools. In 2011, fully 22.6 per cent of American 12th graders reported having used marijuana in the previous month.

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Probably no age cohort uses more pot today than the underaged, which raises especially difficult questions relating to legalization. No one (except perhaps Mr. Trudeau) believes that legalizing pot sales will make it harder for young people to get their hands on the drug. But would legalization really lead to a spike in youth consumption, with devastating health and social consequences?

The truth is that we don't know. The legalization of pot sales in Colorado (and soon Washington state) might show the way, but it will likely be many years before definitive conclusions can be drawn and many reforms made to the marijuana market in those states before they get it right.

That does not mean Canadians can take a wait-and-see attitude. We need to be ready with our own plans, because if the trend takes hold in more states – hello, California – it will be unstoppable. The U.S. pot legalization lobby is professional, organized and well-funded. And Mr. Obama's administration is taking a hands-off approach toward state marijuana initiatives.

"We're now in 1928. It's about to collapse under its own weight," Mark Kleiman, a UCLA policy professor and authority on drug legalization, recently told The New Yorker, likening the current era to the last days of Prohibition.

Perhaps the most compelling reason for legalizing marijuana is the expectation that it will shift control of the pot market from organized crime. But the black market won't die easily. Setting the right price for legal pot (taxes included) will be critical to putting dealers out of business. Yet the price can't be so low as to encourage consumption.

Making any substance legal creates safety liability issues for which governments will ultimately be accountable. In Washington, the state liquor control board will be responsible for running the legal pot market. That suggests Ottawa may need to strike a deal with the provinces before legalizing pot here. The costs of regulating legal weed may match or exceed the costs of policing illegal weed. Will provinces seek compensation for taking on a job they didn't ask for?

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It's a discussion Canadians need to have now – or the 2015 federal election campaign risks degenerating into a petty plebiscite on pot. Even if Mr. Trudeau plays down his support for legalization, Conservatives will ensure every Tory target voter is scared straight about the perils of marijuana and the Liberal plot to make their kids stupid.

Mr. Trudeau started this. He needs to step up with some answers – the sooner the better, for his own sake.