WASHINGTON — Canada’s likely move to completely legalize marijuana next year promises to produce immediate spillover effects in the United States, starting with increased confusion at the U.S.-Canadian border.

“I’m expecting my business to boom,” said Len Saunders, an immigration attorney from Blaine.

With recreational marijuana legal up and down the West Coast, from Alaska to California, he said, more Canadians may let down their guard and admit to U.S. authorities that they’ve used marijuana, reason enough to get foreigners barred from entering the country.

Beyond that, pot retailers and legalization backers say it’s difficult to predict exactly what might happen if Canada, as expected, becomes only the second nation in the world to fully legalize pot for anyone over 18 on July 1, 2018.

Even with such a big move, Jacob Lamont figures the Canadian customers will keep coming to Evergreen Cannabis, his pot shop in Blaine, just a few blocks from the U.S.-Canadian border.

“I enjoy my brothers and sisters from the north — obviously they support my business quite well,” said Lamont, who estimates that Canadian customers make up 60 percent of his business. “They still come down here. They buy a lot of milk, they buy cigarettes and they buy alcohol, because the taxation is so high up there. And I have a feeling they’re going to follow suit with marijuana.”

Oregon Democratic Rep. Earl Blumenauer said it could be a game changer for Congress.

“It completely changes the dynamic,” he said. “Some regard Canada as the 51st state. This is going to make a big difference in terms of adjusting attitudes and accelerating progress. It’s going to help us bring these things to a head.”

Saunders scoffed at the idea that the United States would ever legalize marijuana with President Donald Trump, a teetotaler, in the White House.

“You have a president who not only has an attorney general (Jeff Sessions) who is going to fight drugs, but you have a president who’s never even had a sip of alcohol,” Saunders said.

One of Saunders’ clients, Alan Ranta, 36, a freelance music journalist from Vancouver, B.C., got barred last year as he tried to drive his Toyota Yaris into Washington state. During questioning, he was handcuffed and told a U.S. border guard he had smoked marijuana in the past. Even though he was not carrying the drug with him at the time, Ranta said, he was told that under U.S. law he had committed “a crime involving moral turpitude.”

“It lulls you into a false sense of security when you don’t have anything on you and you’ve done nothing wrong and you’re going to a place where it’s legal,” Ranta said. “You keep thinking, ‘This is crazy, why am I getting in trouble?’”

He figures he was stopped because he and a friend were headed to a music festival, with a banana suit, tutus and a psychedelic top hat visible in the car: “If it’s an electronic music festival, we like to dress up in weird things that we’d never wear day to day.”

Saunders said that even Canadian Prime Minister Justin Trudeau, as a private citizen, could be denied entry since he had admitted to smoking marijuana in the past. Saunders is advising people not to lie to border authorities but to refuse to answer any questions about past pot use.

“Let’s change the question: What if they asked about your sex life? Would you be so forthcoming?” he said. “If you’ve smoked in the past, it’s nobody’s business. If you don’t answer the question, the worst thing they can do is deny you entry. If you answer that question and say ‘yes,’ you are inadmissible for life. It’s a lifetime ban.”

Saunders said Canadians who were barred could apply for a waiver, paying $585 but then having to wait for months for permission to enter the country. The requests are rarely disapproved, but he got his first denial last month for a case involving a 20-year-old Canadian who had admitted smoking pot to an agent. Saunders predicted that it’s a sign of more to come from the Trump administration.

A spokesman for U.S. Customs and Border Protection said any decision by Canada to legalize marijuana would not change anything at the U.S.-Canadian border.