MARK BRODIE: Later this year, Canada will become the second country to legalize recreational marijuana. Uruguay did so in 2013. But will that cause challenges for Canadians coming into the U.S., where pot is still illegal under federal law? With me to talk through this is Henry Chang. He's a partner at the law firm Blaney McMurtry, where he deals with cross-border issues. Henry, what kinds of problems might Canadians face coming into the U.S. once marijuana becomes legal north of the border?

HENRY CHANG: Well, the U.S. laws obviously aren't going to change but the understanding of how the Canadian legalization is going to, you know, interplay with those laws is creating a lot of confusion. So in terms of what the law is doing, is it is, in fact, legalizing marijuana use in Canada but it's still considered a federal offense under U.S. law. It doesn't eliminate prior convictions, so if you're thinking, I got convicted of, you know, possession when I was 18, I'm good to travel to the U.S. now. — it doesn't actually get rid of those old convictions.

BRODIE: But that sounds like that would be an issue for Canadian travelers, regardless of whether the country went ahead and legalized marijuana, right?

CHANG: Yeah because even if it's legal in Canada, it's still illegal in the U.S. and there are still bars under the Immigration Act for individuals that engage in this behavior. So what will change is, if you smoke marijuana after the legalization date, which I believe is October 17th, then it's not going to result in criminal charges in Canada. That's the one change. People are under the... I guess the mistaken belief that they can admit to this to a border officer and it's not a problem anymore. It's true that, you know, the admission of marijuana use after legalization in Canada is not going to result in a bar for the controlled substance frown. There are still other grounds they could use against you. In fact, they probably will.

BRODIE: Is this any different than people coming from any number of countries, which have laws that are different from the U.S. where things are legal there that are not legal here?

CHANG: Well there's case law that says that for an offense to bar you under U.S. law, it has to be a crime in the jurisdiction where it was committed. So, as I said, once they legalize marijuana here, they won't be able to bar you for smoking marijuana under the controlled substance offense. But there are the other offenses, too. So yes, the applicable law in the country where it occurs is very relevant but so is U.S. law. So in terms of them enforcing controlled substance offenses, they're still going to do it. They'll certainly enforce prior convictions in Canada before legalization. Yes, true, if you smoke marijuana in Amsterdam, if you smoke marijuana in Canada after it comes legal, that alone is not a bar, but those other grounds can still bar you.

BRODIE: So if I'm a Canadian and I don't have any prior convictions for smoking marijuana, if I do smoke marijuana after it becomes legal in Canada that does not preclude me from entering the U.S. or coming to the U.S.?

CHANG: No it's not a bar because I did it while I was in Canada where it's legal, at least after the law changes, right? But I could still be barred for those other grounds, like drug abuse or addict or mental defect. Those are not going away.

BRODIE: Do you think that American customs officials are going to be more likely to ask about marijuana usage from Canadians after the drug becomes legal there?

CHANG: They've stated that they're not necessarily going to ask any different questions, but here's the thing — if you smell like marijuana and you're traveling, they're going to ask if you use marijuana, and then the next question will be how long have you been using marijuana. They're going to try to get you to admit that you did it before legalization, and if you do admit that you did it before legalization, you've now made an admission to the commission of a criminal offense and it's as if you were convicted. You're barred.