Eli Lilly has issued a public safety warning regarding potential risks associated with compounded formulations of its GLP-1/GIP agonist Zepbound/Mounjaro (tirzepatide) that are mixed with vitamin B12. The company's testing of such products, obtained on the US market, revealed that mixing tirzepatide with B12 (also known as methylcobalamin, hydroxocobalamin or cyanocobalamin) can lead to a chemical reaction generating significant impurities. The safety and biological effects of these impurities are unknown, as the combination has never been formally studied.

Lilly has notified US regulators of its findings. The company advises patients using any compounded tirzepatide-B12 product, which may be sourced from compounding pharmacies, telehealth services or medical spas, to consult with their healthcare provider to discuss alternative, approved treatment options.

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