Transfusions of blood plasma are used to treat a host of medical conditions, including burns, surgery-induced blood loss, and disorders that prevent a person’s blood from clotting properly. But as Live Science’s Rachael Rettner reports, the Food and Drug Administration has raised the alarm about companies that purport to use blood plasma—specifically the blood of young donors—to combat the effects of aging and several severe ailments.

In a harshly worded statement, FDA Commissioner Scott Gottlieb called out “unscrupulous actors” who claim that infusions of “young blood” can treat conditions ranging from “normal aging and memory loss to serious diseases like dementia, Parkinson’s disease, multiple sclerosis, Alzheimer’s disease, heart disease or post-traumatic stress disorder.”

“There is no proven clinical benefit of infusion of plasma from young donors to cure, mitigate, treat, or prevent these conditions,” Gottlieb said, “and there are risks associated with the use of any plasma product.”

Plasma is the vital blood component that carries nutrients, hormones and proteins throughout the body. Donations of this vital liquid are sometimes called the “gift of life” because of plasma’s critical use in the medical field. But even in approved contexts, there are risks that come with blood transfusions, including circulatory overload, allergic reactions and, less commonly, the transmission of infections. According to Gottlieb, young blood infusions are particularly dangerous because they involve the transmission of large volumes of blood, which in turn heightens the risk of adverse side effects.

In addition to their potential dangers, young blood infusions don’t seem to work. As Gizmodo’s Ed Cara points out, clinical trials have investigated whether blood from young donors can be helpful in treating conditions like Alzheimer’s and Parkinson’s. But to date, Gottlieb says, “there is no compelling clinical evidence on its efficacy, nor is there information on appropriate dosing for treatment of the conditions for which these products are being advertised.”

The FDA didn’t mention any companies by name, but one that has been drawing attention of late is Ambrosia Health, a San Francisco start-up founded by Stanford Medical School graduate Jesse Karmazin. According to Vox’s Chavie Lieber, the company has locations across the United States, and charges $8,000 for a liter of blood drawn from people between the ages 16 and 25. In the wake of the FDA’s caution, Ambrosia announced that it has “ceased patient treatments.”

The thinking behind young blood transfusions stems from a somewhat gruesome experiment conducted in the 1950s, when a Cornell researcher connected the circulatory systems of a young and old mouse, according to New Scientist’s Helen Thomson. The scientist, Clive McCay, found that the old mouse’s cartilage subsequently looked younger than would be expected. More recent research has found that blood from young mice seems to rejuvenate the skeletal stem cells and livers of older mice, and even reverse heart decline in aging mice.

But some researchers involved in this research say that their studies do not lend support to the use of young blood infusions in humans. Irina Conboy, a University of California, Berkeley scientist involved in a 2005 study, told Business Insider’s Erin Brodwin that the positive effects she and her colleagues observed could stem from the fact that the mice were sharing not only blood, but also internal organs.

“When old and young mice are sutured together they share organs too — including their kidneys and all the important filtering organs,” Conboy said. “Imagine you had a new liver. You’d probably see benefits too.”

Interpreting the mice studies as a basis for human young blood infusions, Conboy added, is both incorrect and dangerous. Such transfusions, she told Brodwin, “quite likely could inflict bodily harm.”