The Liberal leader laid out his past marijuana use in a lengthy interview and in an exchange with reporters Thursday in which he made no apologies.
He said he's smoked pot five or six times in his life — including three years ago during a backyard get-together — and never really liked it much.
Now that he's come clean about using pot, he said, he'd like to move on and talk about the hundreds of thousands of people who have a criminal record for it.
Trudeau sought to shift the focus onto his policy of legalizing marijuana when asked by reporters about his drug use and whether it had been a mistake.
"No, it wasn't a mistake," Trudeau told journalists in Quebec City.
"I do not consume cannabis. I am not a big consumer at all. I tried it...
"I've never tried other types of hard drugs. I am not a consumer of marijuana but, yes, I've already tried it. I used it — maybe five or six times in my life."
He said he believes public opinion has moved on and he's confident that Canadians will judge him less harshly than his political opponents.
What matters now, Trudeau said, is ending a marijuana prohibition policy that he says costs law enforcement $500 million a year and has left 475,000 people with criminal records since the Conservatives took office in 2006.
He has said that issue touched his own family.
Trudeau originally made the marijuana admission in a candid interview with the Huffington Post, in which he also revealed that his youngest brother, Michel, was charged with marijuana possession shortly before his 1998 death in an avalanche.
He said he's smoked pot only five or six times in his life, never enjoyed it much, and doesn't do it anymore.
He said he was never known as the pot aficionado in the family. Trudeau also told the website that because he never smoked much he was never the one among his group of friends to buy the weed.
He said he last smoked marijuana about three years ago at his house in Montreal, outside on a patio by the pool.
“We had a few good friends over for a dinner party, our kids were at their grandmother's for the night, and one of our friends lit a joint and passed it around. I had a puff,” he was quoted saying.
Trudeau was elected to Parliament in 2008.
Trudeau's admission will doubtless give more fodder to the Conservatives, who've been pointing to his support for legalizing and regulating marijuana as evidence that he doesn't have the judgment to be prime minister.
The strongest initial reaction came from Justice Minister Peter MacKay.
"By smoking marijuana as a Member of Parliament, Justin Trudeau demonstrates a profound lack of judgment," he said in a statement.
"By flouting the laws of Canada while holding elected office, he shows he is a poor example for all Canadians, particularly young ones. Justin Trudeau is simply not the kind of leader our country needs."
Prime Minister Stephen Harper was more guarded when asked about it, saying only that Trudeau's actions "speak for themselves."
The prime minister has said he's never tried pot but attributes that to his asthma, which would make it painful to smoke anything including cigarettes.
The Huffington Post said the NDP leader's office confirmed that Tom Mulcair has smoked pot "but sent strongly worded emails refusing to say when he last used the drug or where he procured it."
Mulcair's office clarified later Thursday that he had not smoked marijuana since he was first elected to public office — which was in Quebec politics in 1994.
Trudeau had some fun with the issue.
He said on Twitter that he had indeed made a mistake in being so open and was now coming under "vicious attacks" over his other admission in the Huffington Post interview: that he doesn't drink coffee.
More seriously, he said, he doesn't want children to use pot which is why he plans to squash the black market and replace it with a highly regulated trade.
He was forced to admit, however, that in his early days in Parliament his private actions were inconsistent with the drug prohibition policies he espoused at the time.
"Yes," he replied, when asked whether he had done one thing and said another in the past. He said his beliefs on drug policy had changed over time.
He was also asked whether his recent talk about drugs was calculated, as a possible vote-winner. Trudeau said the reaction had been far from unanimous.
Indeed, social media contained a wide gamut of responses Thursday.
There were jokes — many of them about his atypical aversion to coffee. There were laments about his lack of judgment.
And there was one more serious accusation: that Trudeau had been hypocritical in smoking pot privately, while voting publicly for mandatory minimum sentences, in 2009, for marijuana production.
The Quebec City mayor did not appear at the news scrum with Trudeau.
Trudeau said he and Regis Labeaume had a "good meeting" in which they discussed rail transportation, the local port, the airport and the city's infrastructure needs.
-With files from Martin Ouellet and Murray Brewster