07:07 AM EDT, 03/12/2026 (MT Newswires) -- Eli Lilly ( LLY ) Thursday issued a warning about potential safety risks linked to compounded tirzepatide products mixed with vitamin B12 after company testing found a previously unidentified impurity.

According to the company, tests of compounded drugs sold in the US that combine tirzepatide with B12, including methylcobalamin, hydroxocobalamin and cyanocobalamin, detected "significant levels" of an impurity caused by a chemical reaction between the two substances.

The company said the impurity has never been studied in humans. Its effects on toxicity, immune response and how the drug interacts with GLP-1 and GIP receptors are unknown, it added.

Tirzepatide is the active ingredient in Lilly's FDA-approved drugs Zepbound for obesity and Mounjaro for type 2 diabetes.