Eli Lilly Says B12 in Compounded GLP-1s Creates an Impurity. Here's What We Actually Know.
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Eli Lilly released an open letter on March 12, 2026, claiming that compounded GLP-1/GIP products mixed with vitamin B12 contain a chemical impurity with unknown risks, but the company did not share the full data, the identity of the impurity, or its testing methods with the public (Eli Lilly).
Who This Helps
This article is for anyone currently using or considering a compounded GLP-1/GIP medication that includes vitamin B12. If you are new to GLP-1 medications, our GLP-1 basics guide is a good starting point. This article breaks down what Eli Lilly actually said, what the company left out, what independent voices are saying, and what you can do to stay informed and safe.
What Eli Lilly's Letter Actually Says
On March 12, 2026, Eli Lilly published an open letter warning about compounded GLP-1/GIP products that combine the active medication with vitamin B12 (Eli Lilly). Mounjaro and Zepbound are Eli Lilly's FDA-approved brand medications that contain tirzepatide, a dual GIP/GLP-1 receptor agonist (which is the medical term for medications that activate receptors in your body that help with blood sugar and appetite). If you want a deeper look at how tirzepatide and semaglutide compare, we cover that in a separate article.
Lilly's letter claims their testing found "significant levels of an impurity" that forms when tirzepatide and B12 are combined in the same vial (Eli Lilly). The company says this impurity is the result of a chemical reaction between the two ingredients.
Lilly says this impurity is concerning because nothing is known about its short- or long-term effects in humans, its potential impact on how the drug interacts with GLP-1 and GIP receptors, possible toxicity, possible immune reactions, or how the body absorbs and processes it.
The company is asking the FDA to recall all compounded GLP-1/GIP products that include untested additives like B12.
What Eli Lilly Did Not Share
This is where the conversation gets important. Lilly released a press release and pointed to a preprint study posted on medRxiv titled "A Novel, Widespread Impurity in Mass-Compounded Tirzepatide/B12 Products" (medRxiv). A preprint is a study that has not been peer-reviewed. That means no outside scientists have independently checked the work.
Here is what the letter and the preprint do not provide:
- The identity of the impurity. Lilly says they found one but does not say what it is.
- The full methodology. The public cannot see exactly how the testing was conducted.
- Whether the impurity causes harm. Lilly describes the risk as "unknown," not "dangerous."
- Where the samples came from. It is unclear whether they came from licensed compounding pharmacies or from unregulated sellers like medspas or grey-market sources.
The preprint reports that the impurity was found at levels up to 10% of the total polypeptide content in some samples (medRxiv). The researchers used techniques like NMR (a method for analyzing molecular structure) to detect a possible interaction between the two ingredients. But the full details of the analysis have not been released for independent review.
Who Wrote the Study
Every author of the preprint is an Eli Lilly employee (medRxiv). The researchers work in Lilly's regulatory affairs, research labs, and quality control departments at their Indianapolis headquarters. This does not automatically make the findings wrong. But it is a significant conflict of interest that readers should know about. Lilly earned over $36 billion in 2025 from tirzepatide across its Mounjaro and Zepbound brands (Eli Lilly). The company has a direct financial interest in limiting compounded alternatives.
What this means for you: A study funded and authored entirely by the company that sells the brand-name version of a drug should be read carefully. It may be accurate. But without peer review and independent testing, the findings have not been confirmed by anyone outside of Lilly.
What the Compounding Industry Says
The Alliance for Pharmacy Compounding (APC), which represents licensed compounding pharmacies, responded on the same day (APC). CEO Scott Brunner pointed out several gaps in Lilly's announcement.
Brunner noted that Lilly has not shared enough information about where the samples came from, how they were handled, or the nature and level of the reported impurity. He also noted that B12 is a well-studied and widely used vitamin, and that compounding pharmacies have been combining it with GLP-1/GIP medications for roughly four years without a major trend of side effects.
Brunner questioned whether the samples actually came from state-licensed compounding pharmacies operating under federal law, or from unlicensed or unregulated sources. He said the APC would take any verified safety risk seriously, but urged Lilly to share additional data with regulators and the public (APC).
Why B12 Gets Added to Compounded GLP-1 Medications
B12 is one of the most common additives in compounded GLP-1 and GLP-1/GIP medications. There is a practical reason for this.
GLP-1 medications slow down how fast food moves through your stomach (NIH). This helps with appetite and blood sugar, but it can also affect how your body absorbs nutrients. Research shows that patients on GLP-1 medications can see their B12 levels drop by about 15% over 12 months, and roughly 20% of patients may fall below normal B12 levels during that time (Frontiers in Nutrition). Nutrient deficiencies were found in about 12.7% of patients within six months of starting a GLP-1 medication, rising to 22.4% within 12 months (PMC).
Low B12 can cause fatigue, weakness, numbness, tingling, and even memory problems (NIH). Because many people on GLP-1 medications eat significantly less food, getting enough B12 from diet alone can be harder. If you are looking for ways to support your nutrition while on a GLP-1, our guide on supplements that may support your GLP-1 and our meal planning tips are helpful starting points. Compounding pharmacies have included B12 as a way to address a real nutritional concern.
This context matters. B12 was not added to make compounded products seem fancier or more "personalized." It was added because patients on these medications have a documented risk of becoming deficient.
What the FDA's Own Data Shows About Compounded GLP-1 Side Effects
If this impurity were causing widespread harm, you would expect to see a clear pattern in the FDA's side effect reports.
The FDA tracks side effect reports for both brand-name and compounded medications. As of April 2025, the FDA had received 520 side effect reports related to compounded GLP-1 products and 480 related to compounded GLP-1/GIP products (FDA). Many of the side effects reported for compounded products are consistent with side effects seen in the FDA-approved versions of these medications, including nausea, vomiting, diarrhea, and abdominal pain (FDA).
A pharmacovigilance study using FDA's adverse event reporting system did find higher rates of certain issues with compounded products, including preparation errors and dosing mistakes (PubMed). Many of these dosing errors happened because patients were unfamiliar with measuring small doses using insulin syringes or received unclear instructions (FDA).
These are real concerns. You can read more about what the most common GLP-1 side effects are and when they happen in our separate guide. But the documented issues are related to how compounded medications are prepared and packaged, not to a specific chemical impurity caused by B12.
What this means for you: The safety risks that have actually been documented in compounded GLP-1 medications are mostly related to dosing errors, unclear labeling, and unregulated sources. These are problems that a licensed, reputable compounding pharmacy can address. The specific B12 impurity that Lilly is warning about has not been linked to any reported harm.
What About Licensed Compounding Pharmacies vs. Unregulated Sellers
This distinction matters in every conversation about compounded medications.
Licensed compounding pharmacies operate under Section 503A or 503B of the Federal Food, Drug, and Cosmetic Act. 503A pharmacies compound individual prescriptions from a licensed provider and are regulated by state pharmacy boards. 503B outsourcing facilities are registered with the FDA and subject to federal manufacturing standards and inspections (FDA). Both operate within established legal and regulatory frameworks.
Unregulated sellers, grey-market websites, and medspas that sell GLP-1 medications without proper licensing operate outside of these protections. Our article on what grey-market peptides are and why you should care goes deeper into these risks. Compounded GLP-1 products are not FDA-approved as finished drugs, and the FDA says companies cannot market them as "generic" or "the same as" FDA-approved medications (FDA).
If you are using a compounded GLP-1 or GLP-1/GIP medication, knowing where it comes from is one of the most important steps you can take. Our guide on how to evaluate telehealth and compounded GLP-1 providers walks through what to look for. GLP Winner's provider comparison survey can help you compare providers, pricing, and pharmacy sourcing to find a transparent option. You can also learn more about the difference between 503A and 503B pharmacies in our detailed breakdown.
The Bigger Picture
Eli Lilly has been vocal about its opposition to compounding. The company has pushed for legislation that would require compounders to identify all impurities in their products, a process that can take months and could make compounding functionally impossible for medications with short batch shelf lives (APC).
Lilly's letter also describes B12 as one of many "untested additives used in the mass production of compounded GLP-1s under the guise of personalization" (Eli Lilly). This framing groups licensed pharmacies together with unregulated sellers, which is a distinction the APC has pushed back on.
None of this means the impurity finding should be ignored. If there is a real safety concern, patients deserve full transparency. But a press release from a company with $36 billion in annual tirzepatide revenue, backed by a preprint written entirely by its own employees, and released without full data or peer review, raises more questions than it answers.
Should You Be Worried?
Here is what we know right now:
- Lilly says an impurity forms when GLP-1/GIP medication is combined with B12 in a vial. The identity and effects of this impurity are unknown.
- The study behind the claim is a preprint. It has not been peer-reviewed. All authors are Lilly employees.
- Compounding pharmacies have been adding B12 to GLP-1/GIP medications for about four years. There is no documented pattern of harm linked specifically to this combination.
- B12 is added for a real medical reason. Patients on GLP-1 medications are at risk for B12 deficiency because these drugs reduce food intake and slow nutrient absorption.
- The FDA's own side effect data shows that most reported issues with compounded products are related to dosing errors and preparation problems, not a B12-related impurity.
If you are currently using a compounded GLP-1 or GLP-1/GIP medication with B12, there is no evidence right now that you need to stop taking it. Talk to your prescribing provider about any concerns. Ask your pharmacy whether they are a licensed 503A or 503B facility. And stay informed as this story develops.
Final Takeaway
This is a developing story, and it deserves clear-eyed attention. Eli Lilly raised a concern about an impurity in compounded GLP-1/GIP medications that include B12. That concern may turn out to be valid. But right now, the data behind it has not been independently verified, the impurity has not been identified, and no harm has been linked to it. B12 is included in compounded medications because patients on GLP-1 drugs often need it. If you are using a compounded medication from a licensed pharmacy, staying informed is the best thing you can do. Ask your provider questions. Know where your medication comes from. And keep following this topic as more information becomes available.
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