WASHINGTON -- Federal regulators have warned more than 350 medical practices that Botox they may have received from a Canadian supplier is unapproved and could be counterfeit or unsafe.

The Food and Drug Administration said in a letter sent last month, a letter released publicly last week, that batches of the wrinkle treatment shipped by suppliers owned by pharmacy Canada Drugs have not been approved by the FDA and that the agency cannot assure their effectiveness or their safety.

The FDA said Canada Drugs was previously tied to shipping unapproved and counterfeit cancer drugs.

The agency warned doctors about buying drugs from sources other than licensed U.S. pharmacies. It is the fifth warning the agency has made this year about foreign suppliers providing unapproved drugs.

In February, the agency warned 19 medical practices that they had received a counterfeit version of the cancer drug Avastin. On three more occasions the FDA issued similar warnings about counterfeit Avastin and Altuzan, another brand name for the same drug. The alerts were also primarily targeted at drugs distributed by Canada Drugs.

A spokesman for the drug distributor declined to comment when reached by phone Monday.

Health Canada said it was aware of the letter and had been in touch with the FDA as well as RCMP.

However, the department said it currently has no evidence the site is selling unauthorized drugs to Canadians.

"When Health Canada has become aware of internet pharmacies selling unauthorized or counterfeit health products, we have taken appropriate action, including informing Canadians and working with internet service providers to have these sites removed," it said in an email.

It also noted provincial and territorial governments are responsible for regulating pharmacies.

Drug shortages have increased the financial incentives for some pharmacies to provide counterfeit or illegally imported drugs.

The drugs subject to warnings have all been injectable treatments typically distributed through medical practices and not directly to patients.

In October, the FDA ordered operators of about 4,100 websites to immediately stop selling unapproved medications to U.S. consumers. The vast majority of those sites were operated by Canada Drugs. The site was still operating Friday.

Genuine Botox is made by Allergan Inc., based in Irvine, Calif. Avastin is made by Roche Holding AG's Genentech unit.