CBP officials are not planning to go out of their way to interrogate every Canadian traveler about marijuana use. However, other factors may cause them to raise the topic.

“Our officers are not going to be asking everyone whether they have used marijuana, but if other questions lead there — or if there is a smell coming from the car, they might ask,” Owen said. Likewise, marijuana residue, which can linger for weeks inside a car, could be detected by CBP inspection dogs and lead to further questioning, he noted. If asked about past drug use, travelers should not lie, he said. “If you lie about it, that’s fraud and misrepresentation, which carries a lifetime ban,” Owen said.

If a traveler admits to past use of any illegal drugs, including marijuana, the traveler will be found to be inadmissible into the United States. CBP typically will allow them the opportunity to “voluntary withdraw” from the border — or face an “expedited removal.” https://www.politico.com/story/2018/09/13/canada-weed-pot-border-783260