Consumers who try to buy popular weight loss drugs online without a prescription risk being scammed or receiving unsafe products, a new study shows.

About 42% of online pharmacies that sell semaglutide, the active ingredient in Novo Nordisk’s anti-obesity drug Wegovy, are illegal, operating without a valid license and selling medications without prescriptions, according to a study published Friday in JAMA Network Open.

People who shop online for weight loss drugs “face serious consumer risks” of receiving “ineffective and dangerous products,” said Tim Mackey, an author of the report and professor of global health at the University of California, San Diego, as well as the director of the Global Health Policy and Data Institute. The study also included researchers from University of Pecs in Hungary.

Shortages of the popular weight loss medication, which belongs to a class of drugs called GLP-1s, have led to “a black market of illegal knockoffs,” said Dr. Christopher McGowan, the founder, medical director and research director of True You Weight Loss, a weight loss clinic in Cary, North Carolina.