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.

As weight-loss treatments dominate the pharmaceutical market, Swiss drugmaker Novartis AG (SWX: NOVN) is choosing a different path. While competitors like Eli Lilly and Novo Nordisk race to capture the booming weight-loss drug market,

Eli Lilly's experimental next generation weight-loss drug mimics three hormones instead of just one or two and continues to show promising clinical trial results for additional health benefits, the company said.

U.S. pharmaceutical giant Eli Lilly named a new chief financial officer on Monday during a critical time for the company's exploding weight-loss drug market.

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.