Eli Lilly has warned of high levels of an impurity in certain compounded versions of its blockbuster glucagon-like peptide-1 receptor agonist (GLP-1RA) tirzepatide.
Lilly issued the public warning after conducting analytical tests on compounded tirzepatide mixed with vitamin B12, a popular combination offered by telehealth companies, pharmacies, and medical spas.
Tirzepatide is approved in the US under the brand names Mounjaro and Zepbound for type 2 diabetes and weight loss respectively. Compounded tirzepatide combined with B12 – a vitamin that helps nourish the nervous system – is not regulated by the US Food and Drug Administration (FDA).
Lilly said that it had “uncovered significant levels of an impurity” created as a direct result of a chemical reaction between tirzepatide and B12. The drugmaker said “nothing is known” about the short- or long-term effects of this interaction in humans. This not only includes the possibility of limiting the drug’s efficacy but also introducing toxicity and immune reactions, according to Lilly.
“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,” Lilly wrote in an open letter to the public.
However, Scott Brunner, CEO for the Alliance for Pharmacy Compounding, says there is a lack of transparency beyond Lilly's claims.
"Lilly has not provided enough information about the source of the samples, the handling of the drugs, or the nature and magnitude of the alleged impurity for anyone outside the company to evaluate the claim," Brunner commented.
The big pharma company, whose tirzepatide brands generated $36.5bn in global sales in 2025, said it has notified the FDA about its findings. The FDA has already placed a target on the mass-marketing of unapproved drugs. While compounded drugs are custom-made and unbranded medications that contain the same active ingredient as a marketed drug, some providers have taken advantage of regulatory loopholes to mass-market products to patients.
A duality in the obesity market has emerged over recent years in the US, with some patients opting for branded products and others going for the cheaper compounded versions. Both Lilly and Novo have launched a plethora of legal notices against players in the compounded sector, along with pleading with US agencies to clamp down on mass-compounding.
The FDA upped its enforcement activities after telehealth company Hims & Hers announced it would launch a compounded version of Novo’s Wegovy (semaglutide) pill. A legal feud ensued between Novo and Hims, however, this was resolved earlier this week via a partnership. Given there has been no separate update from the FDA, it can be assumed that the agency is still carrying out a regulatory clampdown on companies mass-marketing copycat drugs.
For Lilly’s part, while it welcomes the FDA’s action, the drugmaker said entities continue to mass-compound tirzepatide. “The continued widespread distribution of untested compounded drugs is an unacceptable risk for patients. The FDA’s recent actions are an important step to enforce the law and protect patients, and we urgently call on other regulators and law enforcement to do the same. We also urge the FDA to continue taking action against unlawful mass compounding of tirzepatide that puts the American public at risk, including by requesting a recall of all compounded tirzepatide combined with untested additives like B12,” the company stated.
Editor's note: The third paragraph of this article was updated on 12 March to clarify that Lilly's analysis is on tirzepatide mixed with B12, as opposed to co-administration with the vitamin. A statement from the Alliance for Pharmacy Compounding was also added.
