Canada is to impose restrictions on the export of prescription drugs to US citizens, who pay less for them abroad than they do at home.
The cost of pharmaceuticals in the US is the highest in the world
The US has the highest drug prices in the world, leading many citizens to order their supplies from Canada.
Canadian Health Minister Ujjal Dosanjh said his country could no longer serve as a discount "drug-store" for the US.
US lawmakers are debating laws that could authorise the bulk purchase of medicines from Canada.
The import of prescription drugs - though technically illegal - is widely tolerated in the US.
US citizens have been able to get their supplies from Canada either by crossing the border or by ordering them on the internet.
Drug firm opposition
Mr Dosanjh said he would introduce legislation prohibiting the bulk export of medicines when Canada's supplies were low.
He said US citizens might also be required to get a prescription from a Canadian doctor before they could buy the drugs.
"Canada cannot be a drug store for the United States of America," he said.
"Two hundred and eighty million people cannot expect us to supply drugs to them on a continuous, uncontrolled basis."
US politicians who want their government to end restrictions on the import of drugs from Canada face opposition from large pharmaceutical companies.
Critics of the Canadian government's plans to impose the restrictions say Ottawa is giving in to pressure from the White House and from the pharmaceutical giants.