Canada is a major exporter of high-potency marijuana, according to the man who leads the US war on drugs.
Marijuana is said to be big business in Canada
White House "drug czar" John Walters says new cultivation methods mean marijuana sold today in the US is much stronger than in the 1960s.
"We have a growing problem with the expansion of particularly high-potency marijuana coming from Canada," he said.
Last year the drug produced in British Columbia alone was worth $9bn and most of it was exported to the US, he added.
High-grade marijuana is grown in nutrient-rich solutions rather than soil, and sells for as much as cocaine, according to Mr Walters, who heads the Office of National Drug Control Policy.
Today's pot is on average seven times more potent "than the marijuana baby-boomer parents may remember from their days of use or experimentation", he told reporters.
He said the change had led to a doubling of hospital emergency cases involving marijuana.
Mr Walters added: "Canada is exporting to us the crack of marijuana and it is a dangerous problem."
The Bush administration has criticised Canadian plans to reduce sentences for people caught with small amounts of marijuana.
Canada denies being a major contributor the United States' drug problem.
Ottawa says Washington's own data shows that of all the illegal marijuana seized by US agents, only 1.5% came from Canada.