The U.S. Food and Drug Administration on Monday promised a nationwide crackdown on “unscrupulous” stem cell clinics, following years of reports that some clinics have offered lofty claims of potential medical benefits with scant, if any, medical evidence.

“These dishonest actors exploit the sincere reports of the significant clinical potential of properly developed products, as a way of deceiving patients and preying on the optimism of patients facing bad illnesses. This puts the entire field at risk,” wrote FDA Commissioner Dr. Scott Gottlieb, who was appointed in May. “Products that are reliably and carefully developed will be harder to advance if bad actors are able to make hollow claims and market unsafe science.”

Gottlieb’s statement, published Monday on the FDA’s website, highlighted a warning letter issued to one Florida stem cell clinic on Thursday and the direct seizure of products from another clinic in California on Friday. Gottlieb said the agency is aware of other cases where unproven stem cell treatments have clearly harmed patients, and he promised additional enforcement actions in the coming months.

At the same time, the FDA is planning to unveil new regulatory policy based on medical science and provisions in the 21st Century Cures Act that will allow legitimate research and treatments to be on the right side of the “bright lines” that the FDA is going to draw in the field. Legitimate researchers, including those at smaller companies, will be able to use a new process that is relatively quick and cheap by normal clinical research standards.

“We must put in place the framework to separate the promising treatments from those products that pose significant risks or offer patients little to no chance of benefit,” Gottlieb wrote.

Stem cells are the undifferentiated versions of human cells that have the potential to multiply rapidly and grow into other kinds of cells such as bone, blood and organ tissue. Though stem cell transplants are routinely used in hospitals to replace diseased bone marrow, several smaller clinics have cropped up in recent years offering stem cell injections derived from sources such as body fat to treat myriad health problems, from erectile dysfunction to blindness to cancer.

Critics including University of Minnesota bioethicist Leigh Turner have accused some smaller clinics of taking advantage of patients’ desperation to find new treatments for devastating diseases by charging high rates for unproven therapies.

Turner said some even resort to advertising questionable pay-to-participate research trials on a website run by the National Institutes of Health, even though the treatments carry significant risk and may have minimal oversight. He said he was cautiously optimistic about the FDA’s announcements on Monday, but he wondered why it had taken so long.

“There’s been about a five-year period where these companies have been making a wide array of claims about stem cell treatments, and doing it on their business websites, in news releases, YouTube videos, Twitter. ...,” Turner said. “So I’m a little perplexed as to why it’s taken to this point in 2017 for the FDA to act. But I do think it’s important and it will be interesting to see if there is real enforcement activity.”

Gottlieb’s post said that there are only a “small number” of unethical clinics today and that the new rules will ensure that the “vast majority” of responsible researchers understand the regulations governing the emerging field of regenerative medicine.

It’s not clear to everyone that the FDA has jurisdiction over the whole field.

For example, some stem cell treatments involve cells taken from donors, while others use “autologous” cells taken from the patient’s own body. The FDA said it has jurisdiction over autologous stem cells when they are exposed to more than a minimal level of manipulation outside the body, because that process transforms the cells into a type of biologic drug.

Dr. Ronald Hanson, founder and chief medical officer of OrthoCure Regenerative Orthopedics clinic in Richfield, said some doctors and patients question why the FDA would have oversight over any process in which patients’ own cells are re-injected into their bodies. The FDA regulates medical products, but it does not control physicians’ practice of medicine.

“From the physicians who have practiced for a long period of time, it seems fairly heavy-handed to call somebody’s cells ‘drugs,’ ” Hanson said. “I have to tell you, 100 percent of the patients who I have told that to cringe that their government would classify their cells as drugs.”