Liberal Leader Justin Trudeau says reaction to his admission he smoked pot once after becoming an MP shows the government is "offside from public opinion."

Trudeau said in an interview published Thursday that he smoked marijuana about three years ago, after becoming an MP, and also revealed his brother Michel faced a possession charge before his death in an avalanche.

Trudeau said his brother's legal trouble over a small quantity of marijuana was one of the experiences that influenced what he has called an "evolution" in his thinking on marijuana laws.

Trudeau downplayed his use of pot when speaking with reporters in Quebec City on Thursday.

"I have not taken other drugs, I have been in my past a very rare user of marijuana, I think five or six times in my life that I've taken a puff — it's not my thing. I think I'm in more trouble for admitting that I don't drink coffee on social media today.

"But the reality is that this is an example where we have a government that is completely offside from public opinion from even where the chiefs of police are from what we saw in their announcement yesterday," Trudeau told reporters in Montreal on Thursday.

The Canadian Association of Chiefs of Police, meeting in Winnipeg this week, voted overwhelmingly in favour of reforming Canada's drug laws and to allow officers to write tickets for people found with 30 grams of marijuana or less.

In the interview with Althia Raj of the Huffington Post, Trudeau said the last time he smoked marijuana was about three years ago.

"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," Trudeau said.

"I'm not someone who is particularly interested in altered states, but I certainly won't judge someone else for it," he said in the interview.

But, he added, "I think that the prohibition that is currently on marijuana is unjustified."

Asked about Trudeau's admission, Prime Minister Stephen Harper told reporters Thursday in Rankin Inlet, Nunavut, that the Liberal leader's "actions speak for themselves."

But Justice Minister Peter MacKay said that smoking pot as an MP demonstrated "a profound lack of judgment" on Trudeau's part.

"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," MacKay said in a statement.

Trudeau has come out in favour of the legalization and regulation of pot in recent months after earlier supporting an attempt by the Conservatives to toughen marijuana laws. The government has since passed a law that sets mandatory minimum sentences for possession of marijuana for the purpose of trafficking.

The Huffington Post said it asked all the federal party leaders when they last smoked pot. The Prime Minister's Office said Harper has never smoked marijuana, pointing out he has asthma, and NDP Leader Tom Mulcair's office said the Opposition leader has smoked marijuana but refused to say when.

In an email to CBC News on Thursday afternoon, Mulcair's office said he "has not smoked marijuana since becoming an elected official."

Brother faced possession charge

Trudeau said in the interview his younger brother Michel, who died in 1998 at the age of 23, was facing the charge of marijuana possession after a small quantity of pot was found in his car by a police officer following an accident.

"Mich had charges pending against him when he died... even though it was just a tiny amount," Trudeau said.

Since then, Trudeau said, he has been persuaded by scientific studies and by groups such as NORML Women's Alliance of Canada that regulation and taxation of marijuana is the best way to keep it out of the hands of young people.

He said he still believes marijuana is harmful to young people and would support harsher penalties for selling marijuana near a school.

"I do not see this as a slippery slope…. I see this as an issue of legislators slowly catching up to where public opinion and public behaviour actually is," he told the Post.

After the Huffington Post interview was published, Trudeau took to Twitter in mock regret.

"Realizing I may have made a major mistake in my openness and transparency: vicious attacks coming because I don't drink coffee. #oops," he tweeted Thursday.