Speaking in Vancouver on Thursday, Trudeau said he now backs the legalization — not just decriminalization — of marijuana.
"Decriminalization is a great first step [but] I'm in favour of legalization as well, because we control it, tax and regulate it, we allow for development of a medical marijuana industry."
Trudeau says regulation will also help keep cannabis out of the hands of young people because — just like with cigarettes and alcohol — they will be required to provide proof of age.
"I certainly wouldn't want to encourage people to use it... but in terms of respecting Canadians and their choices... and following where the science leads us is a responsible way of government," he said.
Trudeau admits his opinion has changed over the years, but he says recent studies and evidence indicating that pot is no worse than nicotine or alcohol has swayed him.
"The Conservatives base their approach on ideology and fear. I prefer to base my approach on evidence and best practices and I think that is what Canadians will respond to," he said.
The Harper government has been quick to react.
"I find it quite strange frankly that Mr. Trudeau would be talking about legalization as a priority at this time," said Justice Minister Peter MacKay in Halifax on Friday.
"Our government has no intention of legalization. I would think Mr. Trudeau should look at other areas in which we can end violence and drug use and end this societal ill," MacKay added.
Even the NDP — which has only gone so far as calling for marijuana to be decriminalized — calls it "political pandering".
"[It's a] complicated issue," said federal NDP deputy leader Megan Leslie.
She said Canada would have to consider what impact legalization would have in terms of health, our relationship with the United States, and the economy — especially if it creates tensions with U.S. officials on border security issues.