The major pharmaceutical company says its drugs, also known as GLP-1s, are not meant for those without obesity or Type 2 diabetes.

“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,” an open letter from the company, dated Jan. 4, reads.

Anat Ashkenazi, executive vice president and chief financial officer of Eli Lilly, previously told Fortune that over 110 million patients in the U.S. live with obesity, which corresponds to over 200 health complications.

Mounjaro, approved in the spring of 2022, is for those with Type 2 diabetes, while Zepbound, approved in late 2023, is for those with obesity or who are overweight with an additional chronic condition such as hypertension. Mounjaro and Zepbound are sold in single-dose pens or vials and cost over a thousand dollars per fill without insurance. Both drugs have potential side effects and complications such as thyroid tumors, and stomach and gallbladder problems.

As the surge in popularity for weight loss drugs continues, spawned by influencers and celebrities alike, Eli Lilly is also sounding its alarm on the potential dangers of knockoffs.

The company warns users about brands claiming their products contain tirzepatide, a compound used in Zepbound. Only Zebbound has been authorized by the U.S. Food and Drug Administration to use tirzepatide.