Eli Lilly and Company has issued a public warning about potential safety risks linked to compounded versions of tirzepatide that are being mixed with vitamin B12 and marketed to patients in the United States. The pharmaceutical company said recent internal testing uncovered concerning impurities formed through a chemical reaction between tirzepatide and B12, raising questions about the safety of these unapproved combination products.
According to Lilly, the testing involved compounded products currently being promoted to Americans that contain tirzepatide combined with forms of vitamin B12, including methylcobalamin, hydroxocobalamin and cyanocobalamin. The analysis revealed significant levels of a previously unidentified impurity created when the two substances interact chemically.
The company stated that the presence of this impurity is particularly troubling because there is currently no scientific data on its potential short-term or long-term effects in humans. Researchers do not yet know how the impurity may influence tirzepatide’s interaction with important metabolic receptors, such as GLP-1 and GIP receptors, or whether it could trigger toxic reactions, immune responses, or changes in how the drug is absorbed and processed in the body.
Lilly emphasized that tirzepatide has never been clinically studied in combination with vitamin B12. In addition, pharmacies and companies producing compounded drugs are not required to conduct the same rigorous clinical testing or safety monitoring as manufacturers of FDA-approved medicines. Because of this regulatory gap, adverse events linked to compounded products may go unreported.
The company said patients receiving tirzepatide-B12 products through compounders, telehealth providers, medical spas, or similar outlets may unknowingly be using drugs with unknown safety risks. Lilly has informed the U.S. Food and Drug Administration about the findings and recommends that patients currently using these products consult their physicians about safer, approved treatment options.
Lilly has previously raised concerns about the growing availability of compounded versions of its tirzepatide-based medicines, including Mounjaro and Zepbound. These medications are approved by the FDA for diabetes and obesity management, but unauthorized compounded versions have been widely marketed as alternatives.
The company also welcomed recent FDA statements indicating plans to take stronger enforcement action against the mass distribution of illegally compounded anti-obesity medications. Regulators have repeatedly warned that compounded drugs may carry greater safety risks because they do not undergo the same evaluation for safety, effectiveness and manufacturing quality as FDA-approved treatments.
Beyond vitamin B12, Lilly noted that some compounders are mixing tirzepatide with additional substances such as glycine, pyridoxine, niacinamide and carnitine in attempts to market “personalized” formulations. The company said there is no proven clinical benefit for adding these ingredients and warned that such combinations could create further unknown safety concerns.
Lilly also reported discovering other issues in compounded tirzepatide products, including bacterial contamination, elevated endotoxin levels and additional impurities not found in the company’s approved medicines.
The pharmaceutical company urged regulators to continue cracking down on unlawful mass compounding and called for stronger enforcement measures to protect patients, including potential recalls of compounded tirzepatide products containing untested additives such as vitamin B12.