(Bloomberg) — Shares of Eli Lilly & Co. and Novo Nordisk A/S fell after US regulators established a “green list” of foreign manufacturers who produce raw materials that compounding pharmacies use to make copies of their blockbuster GLP-1 drugs. 

The Food and Drug Administration said in a statement Friday that, in an effort to block the importation of potentially dangerous versions of the active ingredients used in drugs like Wegovy and Mounjaro, it would publish a list of overseas suppliers that appear to comply with the agency’s standards. Compounding pharmacies use these raw materials to make their own versions of the brand-name drugs.

“The agency is aware that some patients are turning to compounded versions of these drugs, which are not approved by the FDA,” it said. The green list includes the active ingredients used in GLP-1 drugs that are produced in facilities that appear to be in compliance with the agency’s standards “to protect patients who use these compounded drugs,” it said.

The list includes over three dozen manufacturers, but their names are redacted. Raw materials known as active pharmaceutical ingredients, or API, from those not on the list may be subject to detention if imported, the agency said.

Lilly shares fell as much as 2.7% on Friday, and Novo’s American depositary receipts declined as much as 2.5%. 

“It is crucial that FDA effectively implement this import alert and take further actions to protect patients from the safety risks posed by taking knockoff drugs made with inauthentic, substandard API that either has the potential to be imported or is already in the US,” Novo said in a statement.

The FDA’s action “is an important first step but more must be done,” Lilly said in a statement. Compounders have already imported large quantities of illicit tirzepatide, the active ingredient in its drug Mounjaro, “and we urge FDA and other regulators to do more to stop unlawful compounding before more people get hurt,” it said.

The news comes as Novo and Lilly have mounted a sweeping but largely unsuccessful legal effort to block compounders from selling alternate versions of their drugs. The knockoffs proliferated when Wegovy and Mounjaro were in shortage, opening a legal loophole that made compounding easier.

Those shortages have since ended, and the brand-name companies contend that the continued compounding of their products is illegal. Compounders disagree, and their medicines are still available.

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