Lilly publishes an open letter on compounded tirzepatide mixed with vitamin B12

Eli Lilly published an open letter on March 12, 2026 warning that compounded tirzepatide mixed with vitamin B12 contains a previously unidentified impurity created by a chemical reaction between the two ingredients ^[1]. The company said it notified the U.S. Food and Drug Administration and is calling for a nationwide recall of compounded tirzepatide combined with additives like B12 ^[1][2].

Tirzepatide is the active ingredient in Lilly's FDA-approved Mounjaro and Zepbound ^[2]. The products Lilly tested were not those brands. According to Reuters, Lilly bought samples from compounding pharmacies, medspas and telehealth networks, and found "significant levels of an impurity that results from a chemical reaction between tirzepatide and B12" ^[2]. The impurity was identified in all 10 samples tested, the company said ^[2].

What Lilly says it found

Lilly's letter says the B12 in these products may be listed as methylcobalamin, hydroxocobalamin or cyanocobalamin ^[1]. The company argues the impurity is concerning because nothing is known about its short- or long-term effects in humans, its toxicity, whether it triggers immune reactions, how the body absorbs and clears it, or whether it changes how the drug interacts with the GLP-1 and GIP receptors ^[1].

Lilly also said tirzepatide has never been studied in combination with B12 in the same solution, and that compounders making these combinations are not required to monitor or report side effects ^[1][4]. "Adding a reactive substance like vitamin B12 without clinical testing or FDA review introduces additional unknown risks," Lilly Chief Medical Officer David Hyman said ^[2].

The letter names other additives Lilly says are being mixed into compounded tirzepatide, including glycine, pyridoxine, niacinamide and carnitine, and says these have no proven clinical benefit for people taking tirzepatide ^[1]. Lilly said it also continues to find bacterial contamination, high endotoxin levels and other impurities in compounded tirzepatide products ^[1]. The company published its analysis in a manuscript titled "A novel, widespread impurity in mass-compounded tirzepatide/B12 products: potential patient safety implications" ^[1][3].

The sources do not report the chemical identity of the impurity, how much of it was present, which sellers the samples came from, or any confirmed patient harm linked to it. Those details are not yet known from the material available here.

Compounders push back

Scott Brunner, CEO of the Alliance for Pharmacy Compounding, told Reuters that Lilly's findings were concerning but incomplete. He said the company would need to disclose details of its sampling process, how the samples were obtained and other specifics before anyone could judge the risk to consumers. "Based on what I've read so far, Lilly isn't telling us enough to make that determination," Brunner said ^[2].

Compounders have argued their products are legal under a narrow provision of federal law that permits compounding when a patient needs personalization for medical reasons, such as adding vitamins or using doses not sold in the branded versions ^[2]. Lilly rejects that framing, saying most sellers put the same additives into every batch to try to evade FDA rules, so the products are not truly personalized ^[1].

Why it matters for patients

A large number of Americans have used compounded tirzepatide, and "B12 blends" are a common way telehealth sellers and medspas market it. This letter is a manufacturer's claim, not an FDA finding or a recall order. As of these sources, the FDA has been notified but has not announced a recall of B12 combination products ^[1][2].

Lilly says people using these products should contact their physicians to discuss alternatives ^[1]. Independently of Lilly's claims, the FDA has repeatedly warned that compounded drugs "pose a higher risk" than approved ones because the agency does not review their safety, effectiveness or quality ^[1][4]. Side effects from any compounded product can be reported to the FDA at 1-800-FDA-1088 or through MedWatch ^[1].

What happens next

Lilly is urging the FDA to request a recall of compounded tirzepatide made with untested additives, and calling on other regulators and law enforcement to act ^[1]. A federal court previously confirmed that mass compounding of tirzepatide must stop, though Lilly says some sellers continue ^[1][4].

The backdrop is a broader crackdown. The FDA issued warning letters to 30 telehealth companies over false or misleading claims about compounded weight-loss drugs, and threatened action against "illegal copycat drugs" ^[2]. Reuters describes those letters as issued in September while linking to a March 3, 2026 report, so the timing is unclear from the sources. In February, Hims & Hers said it would sell its own version of Novo Nordisk's Wegovy pill, then backed away and announced on March 9, 2026 that it would sell Novo's Wegovy and Ozempic instead ^[2].