Eli Lilly & Co. warned that the active ingredient in its weight-loss blockbuster Zepbound could pose a risk to patients when mixed with vitamin B12, a combination sold by US drug compounders.
Lilly tested compounded products and found “significant levels of an impurity” resulting from a chemical reaction between the vitamin and the active ingredient, tirzepatide, it said Thursday.
“People receiving tirzepatide-B12 products from compounders, telehealth companies, medspas, or anyone else should be aware that they may be using a potentially dangerous product with unknown risks,” the drugmaker said in an open letter, citing possible toxicity and immune reactions.
Lilly, like rival Novo Nordisk A/S, faces pressure from cheaper knock-off versions of its obesity and diabetes shots that avert the rigorous regulatory approval process of brand-name or generic drugs.
The compounded medicines were allowed in the US in response to a supply squeeze in the early days of the weight-loss injection boom. The Food and Drug Administration has since declared the shortage over, meaning the discounted copies are no longer permitted.
Even so, compounders have continued to sell their products by exploiting a regulatory loophole that enables them to customize the drug’s formulation — for instance, by adjusting the dose slightly or adding vitamins such as B12.
The testing has detected other critical safety issues such as bacterial contamination and high levels of a toxic component known as endotoxin, Lilly said.
The compounders are mixing tirzepatide with glycine, pyridoxine, niacinamide and carnitine, among other chemicals, creating a range of new and untested combination drugs, according to Lilly. The company urged the FDA to recall all compounded versions of its medicine that include untested additives.
Photo: Photographer: Shelby Knowles/Bloomberg
Copyright 2026 Bloomberg.
The most important insurance news,in your inbox every business day.
Get the insurance industry’s trusted newsletter
