The FDA said it has sent warning letters to four companies for selling unapproved versions of GLP-1 drugs including semaglutide and tirzepatide, the active ingredients in popular diabetes and weight-loss drugs.

The letters were issued to Xcel Peptides, Swisschems, Summit Research and Prime Peptides last week after the U.S. health regulator conducted a review of their respective websites in October.

The websites indicate that their products are labeled as "research use only" or "not for human consumption or clinical use." However, evidence gathered by the FDA establishes that these products are actually intended to be used as drugs for humans.

The agency also sent a warning letter to Veronvy, which offers unapproved and misbranded oral GLP-1 products, including one that claims to be approved by the FDA.

The FDA has observed that the website offers misbranded products that misleadingly suggest that the drug products are approved or endorsed by the agency in some way.

The companies must inform the agency within 15 business days of receiving the letter, outlining the specific actions taken to address any violations. Failure to adequately address the violations may result in legal action including, without limitation, seizure and injunction, the FDA said.

Semaglutide is an active ingredient in Novo Nordisk's Wegovy and Ozempic, while Eli Lilly's diabetes drug sold as Mounjaro and weight-loss drug Zepbound use the ingredient tirzepatide.

Some of the websites were also offering retatrutide, Eli Lilly's experimental next generation weight-loss drug.

The companies have previously sued various medical spas, compounding facilities and clinics for selling products purporting to contain the active ingredients.