Colorado seems like a vision of post-prohibition tolerance for marijuana.

You can buy pot for smoking, dozens of different kinds of edibles, grow your own or qualify as a “cannabis sommelier.” But it’s more complicated than it seems.

Four states have legalized recreational marijuana, and four more voted last November to follow them. But a federal ban on marijuana remains, and doesn’t look like it’s going anywhere any time soon.

READ MORE: Four more states vote to legalize recreational marijuana

For Americans living in pot-tolerant states, it’s smoke ’em if you’ve got ’em, at least for now — U.S. federal prosecutors have been told to focus on other things.

But the conflict has had painful and bizarre results for non-Americans living in or visiting pot-tolerant states. They’ve found that naively answering a question about cannabis at the border (“Sure, I’ve smoked pot, but only in Colorado, where it’s legal”) is enough to bar them from the U.S. for life, abruptly derailing life plans.

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WATCH: U.S. border guards have been barring Canadians from entering the States for answering “yes” to having used marijuana. Immigration lawyer Len Saunders spoke with Geoff Hastings on BC1 about what you should do if you are asked about your pot use.

7:16 Border pot problems predicted to get worse Border pot problems predicted to get worse

“I have some really painful consultations with some really good folks who have been travelling back and forth to the United States for years and suddenly, for whatever reason, they end up in that line of inquiry with border officers,” says Scott Railton, an immigration lawyer in Bellingham, Wash., near the U.S.-Canada border.

Last year, media reported on the case of a Chilean woman who innocently told U.S. immigration officials in Los Angeles that she had smoked pot in Colorado: “It’s legal there,” she said. She found herself in a holding cell, then deported back to Chile with a lifetime ban on entering the United States, derailing a relationship with a U.S. citizen and her plans to attend grad school in Colorado.

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“I have read these sworn statements to officers of the Department of Homeland Security where they say ‘I’ve only smoked it in states where it’s legal,’” Railton says. “And the agency has still gone after them. These individuals aren’t experts on federal versus state law.” Tweet This

Colorado immigration lawyers advise non-citizens to have nothing to do with marijuana — too big a risk, too much at stake.

What happens when a country with an increasingly vindictive attitude to the demon weed — at the national level — and one where it’s totally legal, share an 8,800-kilometre border? We’re about to find out, and the prospect is making some uneasy.

Last week, federal NDP health critic Don Davies called on the government to reach an agreement with the U.S. on border crossings in advance of legalization in July 2018. He predicted that “you’re going to see hundreds of thousands of Canadians denied entry to the United States because they admitted to using cannabis.”

At the moment, Canadians who admit to using recreational pot in Canada can be banned from the U.S. because it’s illegal here.

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After legalization, Railton fears U.S. officials may shift to using laws that allow them to ban marijuana users on health grounds. The only way to be readmitted would be to be certified as “rehabilitated” by a U.S.-approved doctor, though it’s not clear what would happen if you told the doctor that you didn’t see moderate marijuana use as a problem.

“After legalization, I expect that the border will employ different tactics if they’re so disposed on their current path,” he says.

“I recently saw somebody who admitted to using marijuana in their late teen years, and they are middle-aged now. They don’t have a conviction, but that was enough to create a basis for inadmissibility.” Tweet This

Canadians should also be cautious about working for U.S. marijuana businesses, even if they operate legally and openly at the state level. From a federal point of view, they’re all involved with drug trafficking, and a Canadian can be banned for life from the U.S. for admitting to working for one.

“The other trap I’m seeing increasingly come up is people working in the industry. The legalized marijuana industry is a pretty big industry, and sometimes you see Canadian professionals who for this reason or that seek to enter to assist a business in that industry. That can run the business visitor into trouble.”

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Canadians banned from the U.S. can apply for a waiver allowing them to cross, but the process is cumbersome and expensive, and the application has to be restarted from scratch every few years for the rest of the person’s life.

WATCH: Former Toronto police chief and federal marijuana point man Bill Blair said the federal government looked at Colorado and Washington states as examples of how to legalize, regulate marijuana.

1:01 Feds looked at Washington, Colorado for marijuana legalization: Blair Feds looked at Washington, Colorado for marijuana legalization: Blair