Joe Sohm Visions of America/NewscomJoe Sohm Visions of America/NewscomSmoking marijuana will be legal in Canada next month, but anyone who uses the drug could face a lifetime ban from entering the United States. So could anyone who works or invests in the country's legal marijuana industry.

U.S. Customs and Border Protection (CBP) will continue to enforce a policy of absolute prohibition when Canadians try to travel to the U.S., the agency told Politico on Thursday. That means treating marijuana users like criminals and those who work in the marijuana industry like drug traffickers, even if they aren't trying to bring any of the stuff into the states. Politico says CBP did not specify any minimum level of investment that could trigger a ban.

"Our officers are not going to be asking everyone whether they have used marijuana, but if other questions lead thereā€”or if there is a smell coming from the car, they might ask," Todd Owen, a CBP official, told Politico. "If you work for the industry, that is grounds for inadmissibility."

What to do? Well, lying to a border agent is considered fraud and is also grounds for a lifetime ban. "But if an agent asks the question, I suppose they could decline to answer," Lorne Waldman, an immigration lawyer, tells Bloomberg.

While CBP's rules might seem straightforward, there's actually quite a bit of vagueness. Scott Bernstein, senior policy analyst at the Canadian Drug Policy Coalition, tells The Toronto Star that he's worried how far the ban on entry might extend. "That person who owns a mutual fund and maybe doesn't even know where their money is going, are they going to be covered as well?" he asked.

Anyone subject to a lifetime ban from entering the United States can apply for a waiver, though that costs more than $500. The waivers are only granted at the discretion of CBP.

Cannabis companies are already being publicly traded on the Toronto Stock Exchange, and it seems like only a matter of time before enforcing prohibition along America's northern border creates serious problems for residents of both countries.

Consider that, after October 17, someone travelling from Canada (where marijuana is legal) to Washington State or Vermont (where marijuana is also legal) could still be stopped at the border, prohibited from entering the United States, and banned from the country for life. Take that scenario another step. What's a resident of a border town like Blaine, Washington, or Derby Line, Vermont, going to do?