"All it takes is (admitting) using once, that's all it takes, and it doesn't matter how long ago it happened."
Saunders says because of U.S. federal drug laws, Customs and Border Protection agents legally regard marijuana at the border the same way they would react to cocaine and heroin.
"Even though marijuana is legal on both sides of that border, under federal immigration laws marijuana is a controlled substance, and the officers are doing their job if they deny entry to a Canadian if it comes to their attention that they've used it or purchased it," said Saunders.
Border agents can -- and have also banned Canadians who invest in legal cannabis-related businesses in the U.S.
Vancouver B.C. investor Sam Znaimer was given a lifetime ban in May after being stopped at the border and answering Customs and Border Protection agents' questions about his investments in a cannabis-related stock.
"To my shock and horror, I was told I was deemed to be inadmissible to the United States because I was assisting and abetting in the illicit trafficking of drugs, Znaimer said. "It is a huge regulatory over-reach."
Saunders says Canadians who are banned at the border can apply for a temporary waiver, but it costs $585 U.S. money to apply, and can take six months to process.
Some believe the bans are bound to eventually create a chilling effect on cross-border trade.
"I think think there's a lot of people looking at (cannabis) as a great investment," said Canadian business owner Janine Karlsen. "So they might just choose to say, 'You know what? That's getting a little ridiculous, and we don't need to bring our business to the states."'
Saunders agrees, citing the 7,000 Canadian season ticket holders who head to Seattle for Seahawks games every Sunday. "I think you're going to see fewer Canadians going down for games," he said. "Mariners games, Seahawks games, possibly hockey games when there's a Seattle hockey team, and even going shopping, going to outlet malls, and going to Nordstrom."
Blaine business owner Mike Hill, who just opened a landmark lighthouse-inspired Starbucks next to his Chevron station, attracts Canadians for the savings on everything from gas to groceries. "Right now, 90 percent of my customers are from Canada," he said.
"It's tough, because (marijuana) is a legal product on both sides, but you just have to know, federally, at the border, the law is going to be enforced, because that's the agent's job," he said. "I don't notice fewer Canadians coming over, I'm not sure how many people this is affecting, but business here right now, is good."
Saunders says the advice he gives Canadians who are uneasy about answering questions at the border: Silence is golden.
"Your best option as a foreigner coming to this country, if you feel uncomfortable answering that question is to say nothing," Saunders said.
"If you don't answer that question, if you don't cooperate, the worst that can happen is a simple denied entry that day, and so you can try another border another time."