Preprint reports unknown impurity in compounded tirzepatide with vitamin B12

A preprint posted March 9, 2026 reports that researchers found a previously uncharacterized impurity in compounded tirzepatide products that also contain vitamin B12, at levels reaching up to 10% of total protein content ^[1]. The paper has not been peer reviewed ^[1].

The impurity formed through a chemical interaction between tirzepatide and vitamin B12, according to the preprint ^[1]. Researchers reported detecting it in ten compounded samples drawn from multiple compounding pharmacies, med spas and telehealth networks ^[1].

Tirzepatide is the molecule in the FDA-approved products Mounjaro and Zepbound. The samples described in the preprint were compounded products, which are prepared by compounding pharmacies rather than by the manufacturer of those brand-name drugs ^[1].

Up to 10% of total protein content was the highest level reported across the ten samples ^[1]. Which pharmacies, med spas or telehealth networks supplied the samples has not been reported, and neither has the number of samples that contained the impurity ^[1].

How the samples were stored, how old they were when tested, how they were transported, and what concentrations of tirzepatide or vitamin B12 they held have likewise not been reported ^[1].

The preprint includes no data on whether the impurity affects patient safety or how well tirzepatide works ^[1]. It also is not yet known whether the impurity forms in FDA-approved tirzepatide products, whether it can form inside the body after an injection, or whether levels change as a compounded product sits in storage ^[1].

Researchers described the impurity as previously uncharacterized, meaning it had not been identified before this work ^[1].

Why it matters for patients

The finding concerns compounded products that combine tirzepatide with vitamin B12, because the preprint describes the impurity as a product of a reaction between those two ingredients ^[1]. On that description, a compounded tirzepatide product without B12 would not be expected to produce the same reaction ^[1].

The report does not show that the impurity causes harm. No safety or effectiveness data on the impurity are included ^[1], and neither the measured levels nor the reaction itself establishes a health risk on its own.

Because this is a preprint, independent scientists have not yet reviewed it ^[1]. Findings described in preprints can be revised, expanded or withdrawn during peer review.

Whether FDA-approved tirzepatide products are affected has not been reported ^[1]. No statement from federal regulators, compounding pharmacies, telehealth companies or the manufacturer of Mounjaro and Zepbound has been reported either ^[1].

What happens next

The preprint was posted March 9, 2026 ^[1]. No date for peer review or journal publication has been disclosed ^[1], and no regulatory action or product recall tied to the finding has been reported ^[1].