The U.S. Food and Drug Administration said on Tuesday it has sent warning letters to two online vendors for selling unapproved and misbranded versions of semaglutide and tirzepatide, the active ingredients in popular diabetes and weight-loss drugs.

The letters were issued earlier this month to Synthetix, which sells drugs on a website called Helix Chemical Supply, and US Chem Labs after the U.S. regulator conducted a review of their respective websites in October.

The FDA said it found evidence the companies' websites offer the products, labeled for "research use only," for sale in the United States accompanied by claims such as weight loss, reducing the risk of stroke, heart attack, treatment of Alzheimer's disease, among others.

The companies are required to notify the agency within 15 working days of the receipt of the letter, of the specific steps taken to address any violations.

US Chem Labs and Synthetix did not immediately respond to a Reuters request for comment.

Reuters could not find semaglutide and tirzepatide products on Helix, but found semaglutide products on US Chem Labs' website, as of Tuesday afternoon.

Semaglutide is an active ingredient in Novo Nordisk's Wegovy and Ozempic, while Eli Lilly's diabetes drug sold as Mounjaro and its recently approved weight-loss drug Zepbound use tirzepatide. Novo and its biggest rival in the obesity drug market, Eli Lilly, sued several medical spas, weight-loss clinics and compounding pharmacies last year to stop them from selling products purporting to contain the active ingredient.