A number of Canadians seeking to enter the United States for work or personal reasons have been turned away over the years because they admitted to smoking pot. The pot question isn't a question asked of everyone who tries to cross the border, but it can be sparked by a number of things.
One person reported that he was asked about past drug use after U.S. border guards searched his name online and found a link to publications that mentioned drugs. Another person was crossing the border to cover a music festival in his role as a music journalist. He doesn't know why the guards asked him the pot question, but he did answer honestly.
Once he said yes, he had smoked pot in the past, the border patrol reportedly denied him access and banned him from crossing for life. The only way around that lifetime ban, say reports, is by paying a fee of $585 for a travel waiver application. That application still doesn't guarantee entry, either, because the waivers are granted at the discretion of the U.S. government agency.
According to reports, dozens or more of Canadians have had similar experiences and have sought advice from legal counsel. Some ultimately decide to pay the fee for the waiver while others seek to fight the ban through the legal system. No one seems to be able to provide good advice for someone facing such questions at the border, as telling the truth seems to get you banned.
Since border patrol agents can ask any question they want and make on-the-spot determinations, it's hard to offer advice about how to avoid such questions. If you are facing a problem crossing the border, consider seeking professional legal advice about your next options.
Source: CBC News, "Any number of reasons could prompt the pot-smoking question at U.S. border," Peter Zimonjic and Julie Van Dusen,, Sep. 10, 2016