The U.S. Food and Drug Administration (FDA) on Tuesday published letters warning two online vendors to stop selling unapproved versions of semaglutide and tirzepatide, the active ingredients in popular GLP-1 class medications including Novo Nordisk’s powerful weight-loss drug Wegovy.

In the letters sent to Semaspace and Gorilla Healing on Oct. 2, the FDA said the only approved semaglutide products were Wegovy and Novo’s diabetes drugs Ozempic and Rybelsus. It noted that tirzepatide, the active ingredient in Eli Lilly’s Mounjaro, had only been approved for diabetes.

The agency said it was critical that the vendors cease selling the unapproved medicines to protect the public from harm.

"Unapproved new drugs do not carry the same assurances of safety and effectiveness as those drugs subject to FDA oversight. Drugs that have circumvented regulatory safeguards may be contaminated, counterfeit, contain varying amounts of active ingredients, or contain different ingredients altogether," the FDA wrote.

The FDA said the vendors had also violated U.S. law by selling semaglutide and tirzepatide without ensuring customers had prescriptions from licensed healthcare practitioners.

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A sign is seen outside of the Food and Drug Administration (FDA) headquarters in White Oak, Maryland, on August 29, 2020.  (REUTERS/Andrew Kelly/File Photo)

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The agency said that if the vendors did not stop selling the drugs, it might respond with legal action including seizures and injunctions.

Semaspace has shut down its website but Gorilla Healing, which sells products other than obesity medications, is still operating and listing semaglutide 5mg and tirzepatide 10mg for sale at the time of publication.

Gorilla Healing said it had not seen the letter.

In another letter published on Tuesday, the FDA warned www.alphamedstore.com to stop selling unapproved and misbranded opioid drug products, saying opioid addiction and abuse had created an immense public health crisis.

In September, Lilly filed lawsuits against 10 U.S. medical spas, wellness centers and compounding pharmacies for selling products claiming to contain tirzepatide, two months after rival Novo Nordisk sued several similar businesses for selling compounded semaglutide.