Eli Lilly has warned about possible health risks associated with compounded versions of tirzepatide that include vitamin B12, stating that combining the two ingredients can produce a previously unidentified impurity. The warning was detailed in a public letter released by the company, which described results from testing conducted on compounded products.
Tirzepatide is the active ingredient used in Lilly’s medicines Mounjaro, approved for type 2 diabetes, and Zepbound, used for chronic weight management. According to the company, some compounded formulations of the drug contain vitamin B12 as an added ingredient. Lilly reported that a chemical reaction between tirzepatide and the vitamin can generate an impurity during preparation.
The company said the impurity was detected during testing of samples obtained from compounding pharmacies, medspas, and telehealth providers. In total, Lilly analyzed ten samples of compounded products and reported that all of them contained measurable levels of the impurity formed through the interaction between tirzepatide and vitamin B12.
Lilly stated that the presence of this byproduct raises concerns because little information is available about its possible effects in humans. The company said there is limited data regarding the short- or long-term consequences of exposure to the impurity.
The company also noted that there is very little information about whether the substance could influence the way tirzepatide interacts with its biological targets. Lilly indicated that the impurity might affect the drug’s toxicity and immunogenicity, and could also alter how the medication is absorbed into the body, distributed through tissues, and eventually cleared.
In its letter, Lilly said the finding illustrates risks that can arise when additives are combined with complex drug molecules without formal testing. The company wrote that 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 issue is part of an ongoing dispute between drug manufacturers and compounding pharmacies over versions of popular weight-loss and diabetes medications. Lilly has opposed the compounding of tirzepatide, particularly after the U.S. Food and Drug Administration announced in October 2024 that the shortage of the drug had ended. The decision removed the regulatory basis that had allowed routine compounding to help address supply shortages.
In April 2025, Lilly filed lawsuits against four telehealth providers that it accused of promoting and selling products presented as alternatives to its tirzepatide medicines. The legal actions were part of broader efforts by the company targeting compounders, wellness clinics and other organizations that market products claiming to contain the drug’s active ingredient.
Compounding pharmacies and industry representatives have argued that such formulations are permitted under federal law when patients require customized medications. They say compounding can be used when prescriptions call for individualized doses or the inclusion of additional ingredients, such as vitamins.
In January, Arizona-based Strive Compounding Pharmacy filed a lawsuit accusing Lilly and Novo Nordisk of attempting to suppress competition. The complaint stated that actions taken by the companies could disrupt a channel that connects patients who have prescriptions for personalized medicines with pharmacies capable of preparing those treatments.
Lilly said it has notified the U.S. Food and Drug Administration about the impurity discovered during its testing and has called for a nationwide recall of products that contain both tirzepatide and vitamin B12.
“FDA warns that compounded products can be risky for patients because they are not reviewed for safety, effectiveness, or quality,” said David Hyman, chief medical officer at Eli Lilly.
Regulators have also taken steps related to compounded weight-loss drugs. In September, the FDA issued warning letters to 30 telehealth companies for making false or misleading claims about compounded versions of such medicines.
Tirzepatide has become a major source of revenue for Lilly. In 2025, the company reported $36.51 billion in combined sales from Mounjaro and Zepbound.
Tirzepatide Concerns Over Compounded Formulations
Eli Lilly and Company has issued a warning regarding compounded Tirzepatide products that are being mixed with vitamin B12 by some pharmacies. According to the company, altering Tirzepatide formulations could create impurities that may impact safety, quality, and treatment effectiveness.
The pharmaceutical company emphasized that Tirzepatide is carefully manufactured and tested under strict regulatory standards. When compounded versions of Tirzepatide are modified with additional ingredients like vitamin B12, the chemical stability of the medication may change, potentially producing unknown byproducts.
