Maker of Mounjaro writes scathing letter to warn of dangerous ‘chemical reactions’ in knock-off versions of weight-loss drug
The maker of blockbuster weight-loss drugs Mounjaro and Zepbound has issued an urgent warning against knockoff versions of the medications over ‘critical safety issues.’
Pharmaceutical giant Eli Lilly, which makes tirzepatide, the active ingredient in Mounjaro and Zepbound, on Thursday issued a warning because the company claims to have found ‘significant levels of an impurity’ in compounded copycats of tirzepatide.
Compounded medications claim to be customized to meet a specific need that cannot be addressed by FDA-approved, commercially available drugs.
While pharmacies can create compounded drugs, the FDA does not review them for safety or quality, raising the risk of improper dosages and contamination.
Lilly said it found impurities in drugs that combine tirzepatide with vitamin B12, which supports nerve function and the formation of red blood cells. According to Lilly’s announcement, impurities resulted from ‘a chemical reaction’ between tirzepatide and B12.
Additionally, the company warned the ‘risks to patients are unknown because tirzepatide has never been studied in combination with B12 and the compounders making these combination drugs are not required to monitor or report adverse events.’
‘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.’
