Adds comment from pharmacy compounding association in paragraphs 9-10

By Leah Douglas

WASHINGTON, March 12 (Reuters) - Compounded weight-loss drugs that contain vitamin B12 and the main ingredient in Eli Lilly's LLY.N 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 10 samples it tested, the company said.

"FDA warns that compounded products can be risky for patients because they are not reviewed for safety, effectiveness, or quality," Eli Lilly Chief Medical Officer David Hyman said in a statement. "Adding a reactive substance like vitamin B12 without clinical testing or FDA review introduces additional unknown risks."

Lilly said it notified the U.S. Food and Drug Administration of its findings and called for a nationwide recall of products containing both ingredients.

Scott Brunner, CEO of the lobby group Alliance for Pharmacy Compounding, said Lilly's findings were concerning but that the company would need to provide details of its sampling process, how the tested samples were obtained and other specifics to fully assess any health risks to consumers.

"Based on what I’ve read so far, Lilly isn't telling us enough to make that determination," Brunner said.

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.N said in February it would begin selling its own version of Novo Nordisk's NOVOb.CO 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.

(Reporting by Leah Douglas in Washington; editing by Caroline Humer and Bill Berkrot)

((Email: leah.douglas@thomsonreuters.com, Phone: (202) 250-0286))