In a sign of its increasing concern about the marketing of stem cells as cure-alls, the U.S. Food and Drug Administration this week notified a major distributor of such products that they are considered drugs and must have FDA approval in order to be sold legally.

The FDA wrote to R3 Stem Cell following a report this month by ProPublica and The New Yorker that the company’s chief executive officer, Dr. David Greene, was touting products made from birth tissue as therapies for a wide range of ailments despite a lack of scientific evidence. Similarly, the FDA’s review found that the Scottsdale, Arizona, company, which markets stem cells through a network of more than 50 clinics nationwide, encourages patients to use the unproven treatment for dementia, Parkinson’s disease, lyme disease, kidney failure, rheumatoid arthritis and other conditions.

The amniotic stem cell product marketed by R3 needs to be cleared by the agency as “safe, pure and potent” before being marketed to patients, the FDA said in the letter. “We note that your products are intended to treat a variety of serious or life-threatening diseases or conditions,” it continued. “Such unapproved uses raise potential significant safety concerns.”

Greene did not respond to a request for comment late Thursday.

Stem cell purveyors have proliferated across the U.S. in recent years, with more than 700 clinics advertising unproven treatments in 2017, according to a tally by Leigh Turner, an associate professor of bioethics at the University of Minnesota. Patients typically pay at least $5,000 out of pocket for an injection of amniotic stem cells into their joints or spine, while intravenous administrations can cost more than $10,000 a session. Last year, a dozen patients were hospitalized for bacterial infections caused by a contaminated batch of stem cells.

Aiding the industry’s growth was the FDA’s laissez-faire attitude. It allowed manufacturers to decide whether their products should be considered drugs, which would require FDA approval. Rather than undergo the time and expense of clinical trials, most stem cell makers took the stance that their products didn’t count as drugs. The paucity of clinical trials means that there’s little data or research on these therapies. In the past two years, the FDA has begun changing course, notifying more firms that their treatments should be classified as drugs.

The FDA sent R3 Stem Cell what is known as an “untitled letter.” Less harsh than warning letters, which threaten enforcement action, but stronger than “it has come to our attention letters,” untitled letters give recipients an opportunity to correct the problem voluntarily. The administration said it has sent these types of letters to 46 manufacturers and health care professionals in the past year.

“We continue to see companies and individuals use questionable marketing campaigns to take advantage of vulnerable patients and their families with unproven claims about their unapproved stem cell products,” Dr. Ned Sharpless, the FDA’s acting commissioner, said in a statement Thursday. “We will prioritize appropriate regulatory actions against those who place people’s health at risk by promoting unapproved products.”

Formerly an orthopedic surgeon, Greene lost his license to practice medicine in 2009 after surgeries he botched resulted in several deaths and permanent injuries, according to the ProPublica-New Yorker report. He said he had “great outcomes” as a surgeon and the same rate of complications as other doctors who haven’t been sanctioned.

After returning to school and earning an MBA, Greene became a distributor, supplying stem cell products to a network of clinics and promoting them to consumers. At a seminar attended by a ProPublica reporter, Greene extolled amniotic stem cells, made from birth tissues such as placentas or umbilical cords, as a therapy for a vast range of diseases, saying, “Over 85% of patients benefit exceptionally from the treatment.”

Greene told ProPublica that the benefits of amniotic stem cells that he described at the seminar were based on “a lot of success stories” and case studies rather than on large clinical trials. “I don’t claim anything,” he said. “I don’t claim that this is a treatment. I don’t claim that it cures anything. I don’t claim that it’s a permanent fix. All I discuss is maybe, potentially, people can get some improvements from stem-cell care.”