In the United States, Eli Lilly issued a public health warning regarding compounded weight-loss medications that include vitamin B12 and its active ingredient, tirzepatide, used in the company's own products Zepbound and Mounjaro. This warning was made on Thursday, March 12, 2026, as a response to the discovery of an unidentified impurity that poses potential health risks. The impurity was identified in products tested from compounding pharmacies and wellness centers claiming to offer legitimate alternatives to Eli Lilly's medications. Eli Lilly has been actively combating the illegal sale of compounded medications that mimic its offerings. They allege that various compounding entities are marketing unauthorized versions of Zepbound and Mounjaro, leading to legal action against these suppliers. Despite the legality of compounding under certain federal provisions, which allow tailored medications for patients, Eli Lilly’s findings indicate significant health risks. The company’s testing revealed that all ten samples contained notable levels of an impurity produced through a chemical reaction between tirzepatide and B12, raising alarms about its unknown effects on human health. Lilly stated that there is no data regarding the long-term effects, possible toxicity, immune reactions, or absorption of this impurity. This lack of information further accentuates the risks associated with using compounded drugs, which the U.S. Food and Drug Administration (FDA) has warned are not evaluated for safety or effectiveness. The FDA has previously taken action against companies misleading patients about compounded versions of weight-loss drugs, underscoring the potential dangers involved in these products. Following Eli Lilly's announcement, the FDA was notified and a nationwide recall was called for products containing the identified ingredients. The ongoing situation highlights the balancing act between patient needs for custom medications and the potential dangers that unchecked compounded products can present to consumer health.