Here's something that might give you pause if you're using one of those compounded weight-loss drugs: Eli Lilly and Co (LLY) just put out a public warning about potential safety risks in compounded tirzepatide products that get mixed with vitamin B12. And it's not just a routine caution—they found something genuinely concerning.

The company said internal testing detected a previously unidentified impurity that forms when tirzepatide and vitamin B12 interact. Think of it like mixing two chemicals in a lab and getting a third, unexpected compound that nobody's ever studied before. That's essentially what Lilly says is happening in these compounded products.

They evaluated compounded products marketed in the U.S. that combine tirzepatide with vitamin B12—which can also be called methylcobalamin, hydroxocobalamin, or cyanocobalamin. Their laboratory testing revealed significant levels of this impurity created through a chemical reaction between the drug and the vitamin.

The Unknown Factor

Here's why this matters: Lilly says the newly identified impurity raises concerns because its effects in humans are completely unknown. We're talking about a chemical compound that's being injected into people's bodies, and nobody knows what it does.

The company noted there's no data on how this mystery compound could influence tirzepatide's interaction with GLP-1 and GIP receptors—those are the biological targets that make the drug work for weight loss and diabetes. There's also no information about its potential toxicity, immune response, or pharmacokinetics (basically, how the body processes it).

To put it simply: tirzepatide has never been studied in combination with vitamin B12. The FDA-approved versions—Mounjaro for diabetes and Zepbound for obesity—don't contain B12. So when compounders mix them together, they're creating something that's never been tested for safety or efficacy.

There's another wrinkle here: pharmacies producing compounded versions aren't required to track or report adverse events in the same way as manufacturers of FDA-approved medicines. So if someone has a bad reaction to one of these compounded products, it might not get reported through the same channels.

Lilly warned that individuals receiving compounded tirzepatide-B12 products through telehealth platforms, medical spas, or compounding pharmacies could be exposed to potentially dangerous treatments with unknown risks.

The Bigger Picture on Compounding

This isn't just about one impurity. Lilly is using this finding to reiterate broader concerns about what it calls "mass-compounded" versions of its tirzepatide medicines.

The company said it supports recent FDA steps aimed at curbing the distribution of compounded anti-obesity drugs that it believes violate regulatory standards. According to Lilly, some compounders continue to market so-called "personalized" versions of tirzepatide by adding ingredients like vitamin B12, but these formulations are often not customized for individual patients. Instead, they're using identical additives in bulk-produced products.

Think about it this way: if everyone's getting the same B12-tirzepatide mix, that's not really personalized medicine. It's just a different formulation being produced in quantity.

More Than Just B12

Beyond vitamin B12, Lilly says compounders have mixed tirzepatide with other substances including glycine, pyridoxine, niacinamide, and carnitine—creating combinations that have never undergone clinical testing.

The company says its testing has also identified other potential safety problems in compounded tirzepatide products, including bacterial contamination, elevated endotoxin levels, and additional impurities not present in its FDA-approved medicines.

Basically, Lilly is making the case that when you buy a compounded version of tirzepatide, you're not just getting the same drug in a different bottle. You might be getting a chemically different product with unknown impurities and potential contamination issues.

The company is encouraging the FDA to consider recalls of compounded tirzepatide products containing untested additives like vitamin B12. This isn't just a theoretical concern—they want regulatory action.

As for the stock, Eli Lilly shares were down 2.32% at $976.67 at the time of publication on Thursday, according to market data.