2026 Chemistry Study Documented That When Tirzepatide (Mounjaro, Zepbound) Is Compounded With Vitamin B12 the Two Substances Can Chemically Bond and Form a New Molecule Not Present in the FDA-Approved Drug Product, Adding to the Pharmacovigilance Case Against Compounded GLP-1 Formulations Amid the July 30 FDA 503B Bulks List Exclusion Comment Period Close and the Documented 990 Adverse Events for Compounded Semaglutide and 730+ for Compounded Tirzepatide in the FDA Adverse Event Reporting System (FAERS)
A 2026 chemistry study documented that when tirzepatide is compounded with vitamin B12 (a common differentiating additive used by compounding pharmacies to distinguish compounded products from the FDA-approved Mounjaro and Zepbound Eli Lilly formulations), the two substances can chemically bond and form a new molecule not present in the FDA-approved drug product. The finding adds to the accumulating pharmacovigilance case against compounded GLP-1 formulations, which are marketed as bioequivalent to the branded products but frequently contain non-FDA-approved additives whose long-term safety, immunogenicity, and pharmacokinetic profiles have not been characterized in registered clinical trials. Compounding-pharmacy additives often introduce impurities and reaction products that differ from the branded label chemistry. The chemistry-specific finding on tirzepatide + B12 bonding is the type of documented novel-molecule outcome that FDA safety scientists cited in the underlying rationale for the April 30, 2026 proposed rule to exclude semaglutide, tirzepatide, and liraglutide from the Section 503B Bulks List. FDA Adverse Event Reporting System (FAERS) data as of July 2026: 990 adverse events linked to compounded semaglutide, over 730 for compounded tirzepatide. The FDA 503B comment period on the exclusion closed Thursday July 30, 2026.