Moderator, Air Canada; FlyerTalk Evangelist
Join Date: Feb 2015
Location: YYC
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Although marijuana use has been legalized in several US states and in Canada, federally in the US, it's still illegal, and one can be barred for admitting to it. One can also be denied entry for lying to a CBP officer.
There's no guarantee that CBP will ask about it when you go, but there's also nothing stopping them from asking about it.