The federal government is giving "dangerous" advice to Canadians planning to cross the border after marijuana is legalized on Oct. 17, one lawyer says.

Len Saunders, a Blaine, Wa.-based attorney, told HuffPost Canada the government's messaging to Canadians so far has been to tell the truth if they're asked by a border guard whether they've used pot in the past.

But admitting to pot use could lead to a lifetime ban from the U.S., Saunders said, as cannabis is still considered illegal federally. That risk remains even if a Canadian is crossing into a state where pot is legal, since borders are under federal jurisdiction.

Saunders, who testified at a Senate committee in March on the government's cannabis legislation, said the government must warn Canadians about this risk as it approaches the "11th hour" ahead of legalization.

"The advice that they're giving puts Canadians in a position to receive a lifetime bar, and they're not fully explaining that to Canadians," he said.

"I think that's dangerous advice. I think it's inappropriate advice. They haven't thought it through."

Saunders' advice for Canadians is to not answer the question if it's asked.

"You can say 'I don't feel comfortable answering that question' and withdraw your application [to enter]," he said.

That could still make an officer deny your entry, he added, but it can help avoid a lifetime ban.

Canadians must 'reflect' on pot use before travel: PM

During an interview on CBC Manitoba's Information Radio on Tuesday, Prime Minister Justin Trudeau was asked how he would respond if a border guard asked him about his past marijuana use. Trudeau told HuffPost in 2013 that he smoked pot while he was an MP.

"I've never lied to a border guard," Trudeau told CBC's Marcy Markusa, adding that the U.S. has the right to determine who gets to enter its borders.

"I think choosing to partake in marijuana has consequences for individuals, for their lives in different ways, and we're not encouraging that, and that's certainly something that people will have to reflect on if they plan to travel to the United States."