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Eli Lilly said, on March 12, 2026, it had notified the US Food and Drug Administration about an impurity found in compounded tirzepatide products mixed with vitamin B12, based on testing of 10 samples obtained from compounding pharmacies, medspas, and telehealth networks. Lilly said the impurity resulted from a chemical reaction between tirzepatide and B12.
The company’s public warning expands its campaign against compounders marketing products that claim to contain tirzepatide, the active ingredient in Zepbound and Mounjaro. The issue is commercially relevant because some compounders have argued their products are lawful under a narrow federal provision that permits compounding for patient-specific needs, including formulations with added vitamins or doses not available in branded products. Lilly said its findings show that at least one such additive approach may create a new and uncharacterized impurity.
Lilly said nothing is known about the impurity’s short- or long-term effects in humans, including possible effects on receptor interaction, toxicity, immune reactions, or drug disposition. In its letter, the company urged the FDA to continue acting against unlawful mass compounding of tirzepatide and to request a recall of compounded tirzepatide products containing B12.
Source: Reuters, Eli Lilly
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