Canada legalizes pot: Five things to know as Canada becomes the biggest country to support legalization of marijuana

While authorities have placed signs on the Canadian side of the border alerting travelers that it remains illegal to bring marijuana into the U.S., some may not realize they can be barred from crossing for admitting to marijuana use. The same goes for those who tell U.S. authorities they work in the legal Canadian industry. "We're going to see a big problem at the Canada-U.S. border," said Peter MacKay, a former Conservative Party justice, defense and foreign minister. "It's going to be thousands if not hundreds of thousands of Canadians who are suddenly going to find themselves inadmissible to the United States."