Eli Lilly warned on Mar. 12 about potential safety concerns related to compounded versions of its weight-loss and diabetes drug tirzepatide. The company said that some compounded formulations mix tirzepatide with vitamin B12, which can create an impurity whose effects on humans are unknown.
The warning is significant because it highlights the possible dangers patients may face when using altered versions of complex drugs without proper testing or regulatory approval. According to Lilly, there is little information about how this impurity might affect the way tirzepatide works in the body, including its toxicity and how it is processed.
In an open letter, Lilly said the impurity "highlights the risks to patients of haphazardly mixing untested additives with complex molecules like tirzepatide without rigorous testing, clinical trials and FDA approval." The company also stated that these altered products "may pose even greater risks to patients than previously known."
Lilly has opposed compounding practices for some time. In October 2024, the Food and Drug Administration declared that the shortage of tirzepatide was over, removing a key reason for allowing routine compounding. By April 2025, Lilly had filed lawsuits against four telehealth providers accused of promoting and selling unauthorized versions of tirzepatide.
Legal disputes have followed. In January, Strive Compounding Pharmacy filed a lawsuit claiming that Lilly and Novo Nordisk were working together to suppress competition from compounders. Strive argued that efforts by these companies were "cutting off an essential channel between patients with prescriptions for personalized medicines and the pharmacies that could fill those prescriptions." In response, Lilly wrote in its letter: "In reality, these products are not 'personalized' at all," adding that compounders use "untested additivies" under claims of personalization but are actually seeking to "circumvent the law." The company said such changes result in "a range of new and untested combination drugs."
Tirzepatide remains a major product for Lilly. In 2025, sales from its brands Mounjaro (for type 2 diabetes) and Zepbound (for chronic weight management) reached $36.51 billion.
