* Lilly warns of impurity in compounded tirzepatide with

vitamin B12, calls for recall

* Compounders have argued products are legal and safe

* FDA has issued warning letters to compounders for

making misleading claims

By Leah Douglas

WASHINGTON, March 12 (Reuters) - Compounded weight-loss

drugs that contain vitamin B12 and the main ingredient in Eli

Lilly's ( LLY ) Zepbound could present health risks to consumers

due to a previously unidentified impurity caused in the

preparation, the U.S. drugmaker said in a public letter released

on Thursday.

The letter, based on Lilly's testing of samples of

compounded products, is the company's latest move against drug

compounders it says are marketing illegal copies of Zepbound and

its diabetes drug Mounjaro. Both medicines have the same active

ingredient, tirzepatide.

Lilly has sued compounders, wellness centers and other

companies for selling products claiming to contain tirzepatide.

Compounders have argued their products are legal under a

narrow provision of federal law that allows compounding when

patients require personalization due to medical concerns, like

the addition of vitamins or doses not available in the branded

versions.

Lilly's testing of products obtained from compounding

pharmacies, medspas and telehealth networks found "significant

levels of an impurity that results from a chemical reaction

between tirzepatide and B12," according to the letter and a

scientific manuscript on the testing process shared with

Reuters.

The impurity was identified in all ten samples tested by

Lilly, the company said.

"Nothing is known about its short- or long-term effects in

humans, the potential impact on the drug's interaction with the

GLP-1 and GIP receptors, toxicity, immune reactions, or how it

is absorbed, distributed, metabolized, and eliminated," said the

letter, which was posted on the company's website.

"FDA (the U.S. Food and Drug Administration) warns that

compounded products can be risky for patients because they are

not reviewed for safety, effectiveness, or quality. Adding a

reactive substance like vitamin B12 without clinical testing or

FDA review introduces additional unknown risks," said David

Hyman, Lilly's chief medical officer.

Lilly said it notified the FDA of its findings and called

for a nationwide recall of products containing both ingredients.

The FDA in September issued warning letters to 30 telehealth

companies for making false or misleading claims about compounded

versions of weight-loss drugs.

The agency also threatened action against "illegal copycat

drugs" after telehealth company Hims & Hers Health ( HIMS ) said

in February it would begin selling a compounded version of Novo

Nordisk's Wegovy pill.

Hims quickly backed away from its compounded pill and

announced plans this week to sell Novo's Wegovy and Ozempic on

its platform.