Canadian and Chinese officials say they were already helping reduce fentanyl smuggling. Prime Minister Justin Trudeau said fentanyl crossing the northern border is "near zero."

The decrease in fentanyl overdoses, drug policy experts have said, has more to do with public health measures than changes in border policies.

President Donald Trump’s long-threatened tariffs against Canada and Mexico went into effect Tuesday, putting global markets on edge and setting up costly retaliations by the United States’ North American allies.