The US Food and Drug Administration is officially warning consumers that buying young blood infusions to improve their health is not a good idea. It is, in fact, a very bad idea because there is no clinical evidence that the infusions do anything, and the procedure could be dangerous.

“Simply put, we’re concerned that some patients are being preyed upon by unscrupulous actors touting treatments of plasma from young donors as cures and remedies,” write FDA Commissioner Scott Gottlieb and Peter Marks, director of FDA’s Center for Biologics Evaluation and Research in the warning. “Such treatments have no proven clinical benefits for the uses for which these clinics are advertising them and are potentially harmful.”

Related Everything wrong with the young blood injection craze

The idea that infusions of young blood could slow aging has been around since the early 2000s, when studies in mice showed promising results. (Notably, the young and old mice weren’t just sharing blood; they were attached and actually shared organs, too.) The concept really gained traction a few years ago, thanks in part to a rumor that venture capitalist Peter Thiel was interested in the transfusions. As other sites, including Bloomberg have noted, despite the outlandish nature of the claim, jokes about vampirism, and a spoof on the show Silicon Valley, the idea has not gone away and young blood clinics do exist. Notably, a startup called Ambrosia Medical promised transfusions for $8,000 a liter. “It works,” Ambrosia founder Jesse Karmazin told a Mic reporter. “It reverses aging.”

There is almost no evidence that this claim is true. Some of the researchers who did the original young blood studies have shown (again in mice) that old blood hurts more than young blood helps and suggest that it’d be a better idea to instead just study why old blood is harmful. In 2017, a rigorous clinical trial found that young blood given to 18 patients with Alzheimer’s did almost nothing to treat the disease. Not only is this ineffective, but even typical transfusions can be dangerous, leading to a few dozen deaths reported to the FDA each year.

It’s clear that much more research into this “treatment” is needed, though that hasn’t stopped companies like Ambrosia from selling it to people. A Huffington Post investigation found that Karmazin is not allowed to practice medicine in Massachusetts, ran a poorly designed study of young-blood effectiveness that didn’t even have a control group, and has not released any information from said study, despite continuing to take people’s money.

Over and over again, scientists have made it clear that there is no good data to support this practice. The FDA is finally agreeing. So please, if the lack of clinical evidence and questionable practices of the startups weren’t deterrent enough, listen to the FDA on this one.