INDIANAPOLIS (WISH) — Lilly is warning that its blockbuster new drugs to treat diabetes and severe obesity should not be used for ‘cosmetic weight loss.’

The Indianapolis-based drug maker made the plea in an open letter to the public issued Thursday morning.

The letter, published on Lilly’s website, bluntly states the company “stands against the use of its medicines for cosmetic weight loss.“

“Mounjaro and Zepbound are indicated for the treatment of serious diseases; they are not approved for – and should not be used for – cosmetic weight loss,” the letter states. “Lilly does not promote or encourage use of Mounjaro, Zepbound, or any Lilly medicines outside of a medicine’s FDA-approved indication.”

The U.S. Food and Drug Administration in November approved Lilly’s Type 2 diabetes drug Mounjaro for obesity, making official the use of a medicine already widely prescribed off-label for weight loss.

The drug kept the name Mounjaro for treatment of diabetes and is marketed as Zepbound for weight loss.

The letter also warns customers to be wary of compound pharmacies which “mix and combine ingredients to make customized medications based on a specific prescription (e.g., due to allergies to an ingredient). Products claiming to contain tirzepatide, Mounjaro, or Zepbound that are made and/or distributed by compounding pharmacies have not been reviewed by the FDA or global regulatory agencies for safety, quality, or efficacy; are not FDA-approved or approved by global regulatory agencies like Mounjaro and Zepbound; and may expose patients to potentially serious health risks.”

A study released in late November found overweight and obese adults who took the injected Lilly’s Mounjaro lost more weight and were more likely to meet specific weight loss targets than people on a similar competitor, Ozempic.

Sales — and sales forecasts — for Mounjaro and Zepbound have helped push Lilly’s stock price higher in recent weeks.