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To fight off compounding pharmacies, Eli Lilly wants access to medical records of patients who used copycat products.

Eli Lilly & Co. is asking people who took copies of its blockbuster weight loss drug to give the company access to their medical records as it continues to build a case against businesses selling the compounded drugs.

The letter is the latest sign that the battle between the brand-name drug manufacturers and the companies that make and sell the compounded versions has intensified.

“I am gobsmacked that Lilly is seeking confidential patient medical records to build a case against the very therapies that are benefiting those patients at a time when Lilly’s own drugs are not available to those patients,” said Scott Brunner, chief executive officer of the Alliance for Pharmacy Compounding, an industry trade group.

Eli Lilly (NYSE:LLY) is taking the unusual step of asking people who took compounded versions of their drug tirzepatide, marketed as Zepbound for weight loss and Mounjaro for type 2 diabetes, for medical records to examine potential safety concerns.

The Indianapolis-based drugmaker filed a suit in federal court this month, challenging the U.S. Food and Drug Administration’s ruling that the company’s experimental obesity treatment not a biological product.