A Winnipeg-based internet pharmacy, believed to be Canada's biggest, has been ordered by U.S. officials to stop marketing drugs to American customers through 3,700 websites registered in different countries.

CanadaDrugs.com, owned by Manitoban Kris Thorkelson, was sent a warning letter from the U.S. Food and Drug Administration on Sept. 21.

"We continue to see the proliferation of illegal online pharmacies that sell potentially dangerous medicines. FDA targeted those websites selling unapproved and potentially dangerous medicines that can be detrimental to public health," an FDA public affairs officer said in a statement to CBC News.

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Kris Thorkelson, who owns CanadaDrugs.com, was sent a warning letter from the U.S. Food and Drug Administration on Sept. 21. (CBC)

"Some of these products contain active ingredients that are approved by FDA only for use under the supervision of a licensed health-care practitioner or active ingredients that were withdrawn from the U.S. market due to safety issues."

Thorkelson has already been named in an FDA investigation into an international wholesale drug distribution network that has allegedly sold counterfeit versions of cancer drugs Avastin and Altuzan to American oncologists.

CanadaDrugs.com could not be reached for comment today. However, earlier this year a spokesperson said the company doesn't sell Avastin.

Last month's stern warning to stop marketing was part of an international crackdown on the sale of illegal medicines online called Pangea-V. The operation, which spans 100 countries, has also targeted 4,100 online pharmacies around the world, including 3,700 in Canada.

$1M in fake pills seized in Canada

And on Thursday, the RCMP and Canadian Border Services Agency, announced that as part of Pangea-V, 3.75 million units of potentially life-threatening medicines have been seized worldwide, worth $10.5 million US.

In Canada, in the past two weeks more than 138,000 illicit and fake pills were seized — worth just over $1 million Cdn. The FDA confirms that some of those drugs were connected to CanadaDrugs.com.

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Canadian officials have seized more than $1 million worth of counterfeit drugs, including fake Viagra. (CBC)

The RCMP said the drugs that were seized in Canada came from 18 countries, and included anti-depressants, prescription weight loss products and erectile dysfunction drugs.

Tim Smith, president and general manager of the Canadian Internet Pharmacy Association, says Canadadrugs.com has a good track record, but "we'll be very interested to see the response that Canadadrugs gives to the FDA letter and we'll respond accordingly."

Smith said rogue pharmacies should be shut down, but his members run legitimate operations that sell prescription drugs to individual customers in the U.S.

Canada 'a big target' for U.S. authorities

Dr. Brian Liang, head of the institute of Health Law Studies at the California Western School of Law, says Canada is becoming a "big target" for U.S. authorities.

"Canada, because of the online presence, is becoming a big target for us in the United States, for the FDA and the office of criminal investigations. And rightly so," he told CBC News. "There’s still a lot of business up there."

Dr. Stacey Elliot, a clinical professor at the University of British Columbia, says Canadians are also buying fake drugs, and she's worried about what they might be ingesting.

"There could be dangerous substances, there's boric acid, there's arsenic, there's paint, there's plaster," she told CBC News. "There's all sorts of terrible things. Especially the Viagra pills, because they use blue paint to coat them."

Another Manitoba man is also facing charges related to the sale of foreign and counterfeit medicines. Andrew Strempler, one of the first entrepreneurs in the cross-border online pharmacy industry, was arrested in Florida in June on two counts of fraud and one count of conspiracy to commit mail fraud.

The charges relate to a 2005 seizure of drugs shipped by Mediplan Pharmacy, also known as RxNorth.com, which Strempler founded.

The FDA claims many of the drugs promoted online as Canadian actually came from other countries.

An FDA investigation was launched in 2006 after the agency advised consumers against purchasing several prescription drugs from Mediplan.

Strempler has been denied bail and remains in custody.

With files from the CBC's Karen Pauls