Customs and Border Protection officials said: “Although medical and recreational marijuana may be legal in some US States and Canada, the sale, possession, production and distribution of marijuana all remain illegal under US federal law.”
They also noted that anyone who attempts to enter the US in possession of cannabis may have their drugs confiscated.
There is also the possibility that individuals will face fines.
Drew Dilkens, the mayor of Windsor, a town close to the Canadian border, reminded people to be aware of the difference between what is legal and illegal in the two countries.
He said: “We want to make sure that people who come here don’t have a bad experience and that they’re fully informed when they cross the border that the laws are different here.”
Residents in Michigan will soon vote in a ballot to legalise marijuana in October, shortly after Canada does so officially.
It is expected marijuana will be legalised in the US state, meaning the drug will be legal both sides of the US-Canada border.
Washington, Vermont and Maine have already legalised the use of the drug.
However, the legalisation of marijuana will have no impact on the ability for the drug to be taken across the border, as it is controlled by federal officials.
Mayor Dickens said: “This bright red line at the border is something we all need to consider, because federal law will apply at the border, notwithstanding what’s legal or not in either province or state on the other side of that border.”
Immigration lawyer Len Saunders added: “It’s basically black and white; if you admit to a US border official at the US port of entry that you’ve smoked marijuana in the past, you will be barred entry for life to the United States.”
His advice to Canadians would be to remain silent and not answer any questions of this sort.
Nine US states currently permit the recreational use of marijuana, with pressure growing for more states to follow suit.
A further 31 states allow the medicinal use of cannabis.
Marijuana legalisation is gradually gaining bipartisan support, with 62 percent of Republicans under the age of 40 supporting the removal of prohibitions on marijuana consumption.
69 percent of Democrats also support the measure.