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

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.

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.

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.

We recently learned that you took one of our products from a compounded pharmacy,” Lilly wrote in a letter viewed by Bloomberg News. “We would appreciate if you could complete the enclosed form and sign the Authorisation for Release of Medical Information so that we can obtain more details from the treating physician around your experience.