U.S. Food and Drug Administration Commissioner Dr. Robert Califf said Wednesday he is very concerned about the prevalence of compounded and fake versions of highly in demand new weight loss drugs online.

Consumers seeking these medicines are attracted to compounded versions that are sold without proper controls to ensure they get the right active drug, Califf said in a Reuters Newsmaker interview.

"In the context of the total picture of illicit drugs being sold on the internet, it's almost certainly bigger than, much bigger than what we see," he said of such reports to the agency.

Califf said he is also concerned about the "enormous disparity" between people who can get access to the new drugs for weight loss based on wealth or connections to the medical community, and those who do not have those advantages but might have a greater need for the treatments.

Some analysts have forecast the market for weight-loss drugs, currently led by Novo Nordisk's Wegovy and Eli Lilly's Zepbound and Mounjaro, could reach $100 billion a year by the end of the decade.

Earlier this month, the FDA sent warning letters to two online vendors for selling unapproved and misbranded versions of semaglutide and tirzepatide, the active ingredients Wegovy and Zepbound, respectively.