The Canadian Bar Association looks at the example of Uruguay, the only other country that has legalised marijuana. See http://www.nationalmagazine.ca/Articles/Januarry-2016-Web/Legalizing-marijuana-What-about-Canada-s-internati.aspx

They note that, to date, only warnings have been issued to Uruguay and that it is unlikely that any further action will be taken. They also note that, even if the US were to file a complaint against Canada after legalisation, "the legal mechanisms involved here are more symbolic than practical" and that there is no real way to enforce a ruling.