The Food and Drug Administration issued a warning Tuesday about new businesses offering blood transfusions from young donors.

Some companies, according to Yahoo News, "charge thousands of dollars to inject older patients with infusions of blood plasma from young donors."

In December, the Huffington Post detailed how one such company claimed these transfusions could reverse aging and provide something "pretty close" to immortality. None of those claims had any proof.

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“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,” the FDA said. “Such treatments have no proven clinical benefits for the uses for which these clinics are advertising them and are potentially harmful.”

The FDA statement, which is credited to Commissioner Scott Gottlieb and FDA director Peter Marks, warns consumers these treatments have not been tested to show they actually help patients. In fact, the FDA says, the transfusions may hurt the recipient.

“Plasma administration is not without risks,” the men wrote. “The more common risks are allergic reactions and transfusion-associated circulatory overload.”

Less common risks include acute lung injury, infectious disease transmission or circulatory overload, the FDA warned.

Plasma is the liquid part of blood, containing blood-clotting proteins.

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The FDA said there are a few instances where a plasma transfer has proved to be safe and useful. For example, patients whose blood is unable to clot because of illness or medication might receive a plasma transfer.

In such a case, the benefits (helps the patient’s blood to clot) outweigh the risks (allergic reactions, etc.). However, even then, the patient still faces the same risks inherent to transfusions.

“As a general matter, we will consider taking regulatory and enforcement actions against companies that abuse the trust of patients and endanger their health with uncontrolled manufacturing conditions or by promoting so-called ‘treatments’ that haven’t been proven safe or effective for any use,” the FDA said.