WINDSOR, Ontario (Reuters) - U.S. Senator and Democratic presidential hopeful Bernie Sanders joined diabetes patients as they drove across the border on Sunday to buy cheap insulin in Canada, highlighting his argument in favor of federally negotiated prescription drug prices.

Sitting in the back of a chartered bus organized by members of advocacy group Insulin4All, Sanders spoke to families who said they go to great lengths to ration their insulin because of high U.S. costs, often putting their health at risk.

“We should be doing what the Canadians do, and that means sitting down with the drug companies and negotiating a price,” he said.

Democrats seeking to take the White House in 2020 see President Donald Trump as increasingly vulnerable to criticism on healthcare costs, since his administration has failed to push through several initiatives to lower drug prices.

Kathy Sego and her son Hunter, 22, picked up six months worth of insulin from the Olde Walkerville Pharmacy in Windsor, Ontario, across the river from Detroit, spending a little more than $1,000.

“That’s still less than what I pay a month in the United States,” Sego said, fighting back tears.

Stephanie Odette, 30, said she started to ration her supply when she was in college.

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“I was hospitalized 74 times ... which has got to be some sort of a record,” she said.

Drug manufacturers say they have to raise U.S. list prices to help offset steep rebates they must offer to get drugs covered by insurance plans.

The U.S. government should set drug costs based on the average prices in six other countries, including Canada, Sanders said. He also called again for the United States to allow healthcare providers and others to import drugs to reduce costs.

“I believe we should be able to import into the United States from Canada, and from other countries, FDA-approved medicine, which would substantially lower prices,” he said.

While manufacturers ship their drugs across borders, they set different prices in different countries, and other companies are generally not allowed to import drugs meant for other markets.

Several U.S. states have passed laws to allow large-scale imports, but shipments would not be legal without federal approval. The Health and Human Services secretary said in early July the government was looking at the issue.

Reuters reported earlier this month that Canadian officials have warned the United States they would oppose any imports that raise drug prices or cause shortages in Canada.