Justin Trudeau's enthusiastic embrace of the legalization of marijuana has fired up the debate over Canadian drug laws and exposed stark differences among major political parties on the way to treat the country's numerous pot smokers.

The Liberal Leader, who had expressed reservations about loosening up Canada's marijuana regime in the past, said his position has "evolved," placing him in the camp of those who would see cannabis regulated and taxed by the government, and sold legally.

The stand places the Liberal Party on a collision course on the road to the 2015 elections with the Conservative government, which is solidly in favour of the status quo, and the NDP, which would only go as far as decriminalizing the possession of small quantities of marijuana.

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Mr. Trudeau is the first leader of a major Canadian political party to advocate for legal pot – and he takes that position as his party and the NDP fight to capture the progressive side of the political spectrum in the next two years.

At the end of a tour of British Columbia, where illegal pot crops abound and the pro-marijuana culture has flourished, Mr. Trudeau said he does not advocate drug use. Still, he added that regulating and taxing marijuana would keep it out of the hands of young people and allow for the development of the medical marijuana industry.

He argued the current approach to drugs is not working and that Canada would do well to follow the lead of Washington State and Colorado, which voted in favour of marijuana legalization last year.

"Listen, marijuana is not a health food supplement, it's not great for you," he told reporters Thursday.

Still, he said studies have shown it's not worse for people than cigarettes or alcohol, and that he is now willing to go further than decriminalization, which entails smaller penalties, such as a fine, for drug possession.

"I have evolved in my own thinking," Mr. Trudeau said. "I was more hesitant to even decriminalize not so much as five years ago. But I did a lot of listening, a lot of reading, and a lot of paying attention to the very serious studies that have come out and I realize that going the road of legalization is actually a responsible thing to look at and to do."

The Conservative government stated that it will remain opposed to any loosening of Canada's drug laws, which it has toughened since coming to office in 2006.

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"These drugs are illegal because of the harmful effect they have on users and on society, including violent crime. Our government has no interest in seeing any of these drugs legalized or made more easily available to youth," the Prime Minister's Office said in a statement.

Employment and Social Development Minister Jason Kenney said on Twitter that Mr. Trudeau's position was "irresponsible," and the Conservative Party released a series of statements from police officials and health experts against the legalization of marijuana.

The NDP blasted current and previous governments for failing to modify Canada's drug laws despite the findings of the Le Dain Commission, which called for the decriminalization of marijuana possession in 1972.

NDP MP Rosane Doré Lefebvre said the current war on drugs "is not working" and her party would not send anyone to jail for having a small quantity of marijuana.

"We are in favour of prevention instead of the Conservative Party's punitive approach," she said in an interview. "Something has to change because the status quo is unacceptable."

Mr. Trudeau fuelled the debate this week as he attended a public event in Kelowna, B.C., where he noticed someone holding a sign that called for the decriminalization of cannabis.

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"I'll take that as a question," said Mr. Trudeau, on a video that was posted on YouTube.

"I'm actually not in favour of decriminalizing cannabis, I'm in favour of legalizing it, tax and regulate," he said to applause. "It's one of the only ways to keep it out of the hands of our kids, because the current war on drugs, the current model, is not working."

Mr. Trudeau was a lot more hesitant as he discussed the issue at the Liberal convention in 2012, expressing doubts even with decriminalization.

"One of the things that pot does is disconnects you a little bit from the world, and it's not great for health," Mr. Trudeau said in an interview with the Red Dot Project.

In another comment, this time to Maclean's in 2010, Mr. Trudeau said the strength of today's cannabis was a potential problem.

"It's not your mother's pot," Mr. Trudeau was quoted as saying. "I lived in Whistler for years and have seen the effects. We need all our brain cells to deal with our problems."