As the Trudeau government prepares to draw up legislation that would legalize marijuana for recreational use, leading policy experts in the United States have some pointed advice for Canada: Rules are important, but cultivating unwritten social standards around how people use the drug are just as crucial.

In states such as Colorado and Washington, where prohibition of cannabis has been lifted, lawmakers have seen recreational marijuana use soar. While that has pumped welcome tax dollars into government coffers, it has also led to problems with public consumption, overuse and intoxicated driving.

"Do it cautiously," Mark Kleiman, a professor of public policy at New York University, said at an international conference on cannabis policy on Sunday. "A bunch of the mistakes that are happening in the U.S. don't have to happen in Canada."

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In several states and countries, the debate over marijuana legalization is no longer over whether to do it, but how to do it properly, Mr. Kleiman said. His comments came on the first day of the conference, where policy leaders from around the world gathered to discuss how to properly roll out legalization.

The two-day cannabis summit is scheduled to discuss Canada's plans for legalization in depth on Monday, an indication that the Liberal government's intentions are being watched closely by policy makers around the world.

"You want people who want [marijuana] to be able to get it, but you don't want anybody to be pushing it at them," Mr. Kleiman said after addressing an audience that included academics, doctors and representatives from top research firms such as Rand Corp., which has been tapped in the U.S. to study the impact of legalization on the economy and society.

Mr. Kleiman and Jonathan Caulkins, a professor of public policy at Carnegie Mellon University in Pittsburgh, are leading a push for governments to think of marijuana less as a commodity and more as sugary foods or gambling – "temptation goods" that must be treated carefully so that overuse doesn't end up burdening society.

To do this, legislators must think about more than just laws, but about fostering social habits that can promote responsible use, Mr. Caulkins said. Alcohol consumption has been shaped this way – with varying degrees of success – over the past several decades, using a mixture of legislation and social influence.

"Some people, as a social norm, don't drink alcohol before lunch. That's not a law, it's not a regulation, it's just a social norm," he said.

"So if we are honest about the potential to lose control of cannabis use, then we might as a society – outside of the political process – want to try to create social norms to control use."

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Doing so is easier said than done, Mr. Caulkins admitted. There are only so many ways to shape consumer habits beyond writing rules, such as using advertising and public service announcements. Now that governments such as Canada are pursuing legalization, though, those strategies will be needed to prevent larger problems, particularly when laws alone are not enough.

"Most people now know that it's wrong to drive while intoxicated by alcohol; a distressing proportion of cannabis users think it's just fine to drive soon after using marijuana," Mr. Caulkins said, citing survey data in the United States.

Since legalization began in some states, marijuana use has risen sharply. In 1992, there were 10 times as many daily or near-daily alcohol users as there were daily marijuana users in the United States, the conference heard. But the pattern of marijuana use has understandably accelerated with easier access to the product. Daily and near-daily use of marijuana compared with alcohol "has fallen below two to one," Mr. Caulkins said.

"Most people are very happy with their cannabis use, but I think there will be a substantial number of people who are going to be harmed by this corporate, commercialized regime that we are creating," he told the conference.

Companies that produce marijuana will be inclined to market their product aggressively in search of profit, so jurisdictions will need to be wary about the impact on recreational consumption, he suggested.

"The children of today are going to go through their adolescence and young adulthoods living in worlds where marijuana is cheap, ubiquitous and heavily marketed," Mr. Caulkins said. In addition to writing rules around legalization, "it is our obligation to construct informal social norms to take the place of those formal social controls that are being removed," he said.

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Should Canada legalize marijuana, it would join Uruguay in permitting cannabis to be broadly purchased and consumed for non-medical and non-religious reasons, surpassing jurisdictions such as the Netherlands, which allows use, but only in designated cafés.