Canada will this week become the second country in the world to legalize recreational marijuana, but as they negotiate a patchwork of new legislation and inconsistent enforcement, smokers may soon find that their enjoyment of weed is still blunted.

New rules governing cannabis use are different in each of the country’s 10 provinces and three territories, and campaigners warn that experimentation could still result in hefty fines – or even arrest.

“There will be more laws around the cannabis plant after legalization than there were before,” said Akwasi Owusu-Bempah, a professor at the University of Toronto. “I don’t think the average Canadian is aware of that.”

The prime minister, Justin Trudeau, was elected on a manifesto promise to follow the example of Uruguay and legalize cannabis, arguing that the move would cut the estimated C$6bn ($4.5bn) in profits pouring into the black market.

As of Wednesday, Canadians aged 18 and over will be able to legally purchase the drug for recreational use. (Medical marijuana has been legal since 2001.)

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Exactly how they will be able to buy it will vary from province to province: Nova Scotia, (population 940,000) will have 12 stores, run in conjunction with the province’s liquor board; British Columbia (population 4.6 million) will have just one. In Ontario – Canada’s most populous province – it will initially only be available online.

“It’s amazing that Canada has taken this position, setting the stage for the world to watch as we show how cannabis legalization is a good thing,” said Robin Ellins, the owner of a cannabis accessory shop in Toronto. “We’ve spent a quarter of a century advocating for legalization. And now, it’s here.”

But regulations rushed into place to govern the legal market could have jarring and unintended consequences, said Abby Deshman of the Canadian Civil Liberties Association.

“We’re legalizing the industry, but criminalizing a lot of the aspects around the use of cannabis,” she said.

Only purchases from officially recognized stores will be legal: someone selling a few ounces to a friend could still face fines or even jail time.

Giving marijuana to a minor remains illegal, so an 18-year-old sharing marijuana with a 17-year-old could in theory face a maximum sentence of 14 years in jail.

“The danger in this is that people are going to go out and think that they’re using a legal substance and will use it in a variety of ways that may seem innocuous, but could result in criminal charges,” said Deshman.

Meanwhile, the government has shrugged off calls for an amnesty for those prosecuted under the previous legal framework: more than 15,000 have been charged over marijuana-related offences since Trudeau was elected in 2015.

“I want to see social justice happen to those who were criminalized for possession of the plant,” said Ellins. “I want to see them go back and expunge that.”

Studies have found that black and indigenous residents have faced disproportionate charges and sentencing in Canada’s criminal justice system, and legal experts warn that racial disparities will persist after legalization.

“Because our policing practices are racialized anyway, there’s no reason to think that’s going to change after legalization,” said Owusu-Bempah.

“We’ve got evidence from a number of American states that the racial disparity in arrests for things that remain illegal actually grow after legalization or decriminalization.”

Even with more minor aspects of the law, the rules vary dramatically across the country: in Ontario, people will be free to smoke or vape marijuana anywhere they can legally consume tobacco, but in Saskatchewan, public consumption of cannabis will incur a $200 fine; in Manitoba the penalty will be $672.

Travelling with marijuana will not be straightforward: in Manitoba, it must be kept in the car’s trunk. In Prince Edward Island, it can be kept in open packaging, but out of the reach of the driver or passengers. But in Canada’s north, residents of Nunavut will be barred from carrying it any vehicles.

Stiff penalties will be imposed for anyone caught with more than five nanograms of THC – the psychoactive component of marijuana – in their blood. But critics argue the limit is arbitrary and not backed up by science and warn that medical marjiuana users, who develop higher natural THC levels in their blood, could end up facing severe punishment.

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The stakes are potentially much higher at the border with the US, where marijuana possession and trafficking remains a federal crime. The US border agency was recently forced to clarify that Canadians who work in the burgeoning legal marijuana sector will be allowed to travel to the US, after several were reportedly turned back at the frontier.

Meanwhile, some police officers are still wondering if they themselves will be able to use marijuana while off-duty: in Vancouver, officers have been told to treat it like alcohol, as long as they show up for the job sober. But members of the Toronto police and the Royal Canadian Mounted Police must wait 28 days after consuming the drug before they are considered fit for duty.

Many of the laws reflect a cautious approach by both the federal and provincial governments – and a recognition that the rollout and enforcement will be a learning process for every group involved, said Deshman.

One thing is certain, she added: “There will be legal challenges.”