Eli Lilly, the pharmaceutical powerhouse behind the popular weight-loss medications Mounjaro and Zepbound, has sounded an alarm about counterfeit versions of these drugs, citing “critical safety issues.”

On Thursday, Eli Lilly, the company responsible for producing tirzepatide, the active component in both Mounjaro and Zepbound, issued a cautionary statement. The company reported discovering “significant levels of an impurity” in unauthorized replicas of tirzepatide.

These knockoff medications are often marketed as customized solutions designed to meet particular needs not addressed by FDA-approved, commercially available treatments.

Although pharmacies are permitted to create compounded drugs, these versions are not subject to FDA review for safety or quality, which increases the potential for incorrect dosages and contamination.

Lilly identified impurities in drugs that mix tirzepatide with vitamin B12, which is essential for nerve function and the development of red blood cells. The company noted that these impurities result from “a chemical reaction” between tirzepatide and B12.

Moreover, Eli Lilly highlighted that the “risks to patients are unknown” because the combination of tirzepatide with B12 has not been studied, and those producing these compounded drugs are not mandated to monitor or report any adverse effects.

‘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 said. 

The warning comes after the FDA said last month it would ‘take decisive steps’ to restrict the sale of non-FDA-approved compounded weight-loss drugs to ‘safeguard consumers from drugs for which the FDA cannot verify quality, safety or efficacy.’

Mounjaro and Zepbound maker Eli Lilly has warned against compounded copycat versions of their weight-loss durgs (stock image)

Lilly said: ‘Although the FDA and a federal court have confirmed that mass-compounding of tirzepatide must stop, some entities continue to do it, claiming to offer “personalized” versions by adding untested additives, such as B12, to compounded tirzepatide.

‘In reality, these products are not personalized at all. Most sellers put the same untested additives in all their tirzepatide knockoffs to try to evade FDA regulations. 

‘Our testing results of these so-called “personalized” compounded tirzepatide products show they may pose even greater risks to patients than previously known.’

November 2025 data from Kaiser Family Foundation shows one in eight Americans has tried a GLP-1 such as Mounjaro, Zepbound or Ozempic, for weight loss, diabetes or other conditions such as polycystic ovary syndrome (PCOS). 

This figure has doubled from roughly six percent measured in a February 2024 Gallup poll.

Compounded GLP-1 drugs became popular in the last few years as drugs like tirzepatide and semaglutide – the active ingredient in Ozempic and Wegovy – have faced shortages. 

They are also often much cheaper than traditional GLP-1s, costing $130 and $450 per month compared to upwards of $1,000 for classic versions without insurance. 

In addition to the FDA’s warnings, Eli Lilly and Ozempic maker Novo Nordisk have pursued legal action against compounded distributors such as Hims.

Earlier this month, Hims announced it would stop marketing these drugs in a deal with Novo Nordisk. 

It’s unclear exactly what impurity resulted from combining tirzepatide and vitamin B12, but Lilly believes its findings highlight ‘the risks to patients of haphazardly mixing untested additives with complex molecules like tirzepatide without rigorous testing, clinical trials and FDA approval.’

‘Our testing results of these so-called “personalized” compounded tirzepatide products show they may pose even greater risks to patients than previously known,’ Lilly said in its announcement (file photo)

While B12 is generally considered safe, high doses can be dangerous for people with chronic kidney or liver diseases as their bodies cannot effectively clear it. And according to Mayo Clinic, there is no solid evidence that B12 helps with weight loss.

‘B12 is just one of the many untested additives used in the mass production of compounded GLP-1s under the guise of “personalization,”‘ Lilly said.

Lilly also claims several companies selling compounded GLP-1s have mixed tirzepatide with vitamins B3 and B6, as well as the amino acid compound carnitine, which have unknown effects on the drugs’ safety and effectiveness.

‘These additives have no proven clinical benefit for patients taking tirzepatide, and the resulting combinations introduce unknown risks for patients,’ Lilly said. 

‘We also continue to find other critical safety issues in compounded tirzepatide knockoffs, including bacterial contamination, high endotoxin levels, and other impurities that are not present in Lilly’s FDA-approved medicines.’

Additionally, the FDA previously noted it has collected reports of compounded GLP-1s not being refrigerated correctly or being made with poor-quality ingredients. Adverse effects from drugs handled under these circumstances include redness, site swelling, pain and a red lump at the injection site. 

Lilly said: ‘The continued widespread distribution of untested compounded drugs is an unacceptable risk for patients.

‘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.’