A new law will come into effect on October 17, 2018, which will allow Canadians to possess and produce cannabis (marijuana) for personal use.
However, the Cannabis Act is not carte blanche; the legislation still makes it an offence to possess illicit cannabis:
illicit cannabis means cannabis that is or was sold, produced or distributed by a person prohibited from doing so under this Act or any provincial Act or that was imported by a person prohibited from doing so under this Act.
If you are charged with or convicted of a cannabis offence in another country (where it is still illegal to possess cannabis), then you could be inadmissible to Canada even though it will be legal to possess it here. This is because the illicit cannabis definition could apply to the acts committed in that country. Further, you need not have been convicted of an offence to be inadmissible; just ‘committing’ an act which would be an offence under law could be enough to make you inadmissible to Canada. Therefore, if you admit to a Canadian border official that you have smoked weed in a jurisdiction where it was not legal to do so, you could be refused entry to Canada.
In the United States, though some states have legalized cannabis, it is still against federal law to possess it. As such, all cannabis possession in the United States arguably constitutes possession of “illicit cannabis”. This would make every United States resident who has smoked marijuana inadmissible to Canada. This is much harsher than the previous legislation.
It is also important to know that you will not be able to bring cannabis to Canada once it is legal to possess it here. This will still be considered ‘illicit cannabis’. You can read more about this here: https://travel.gc.ca/travelling/cannabis-and-international-travel
You can read more about the new legislation here: https://www.canada.ca/en/services/health/campaigns/introduction-cannabis-act-questions-answers.html
Finally, Canadians themselves should also be wary of the implications of smoking cannabis after it’s been legalized, and then seeking entry to the United States. Since it is still illegal on the federal level in the US, an admission to having legally smoked cannabis in Canada could mean a bar to entry to the United States. For more see the following article: https://www.cbc.ca/news/politics/smoke-pot-us-border-1.4718571