Four years after Texas facilitated the medical use of stem cell therapy, federal regulators have begun a new crackdown on clinics that offer the pricey, unapproved treatment.

The publication Stat reported Monday that the Food and Drug Administration had issued draft guidelines clarifying that adult stem cells used in most clinics are drugs and must undergo a rigorous approval process before patients use them. It also reported the FDA sent a warning letter to stem cell clinics in California, New York and Florida in late December about their unlicensed use.

"The FDA is concerned that the hope that patients have for cures not yet available may leave them vulnerable to unscrupulous providers of stem cell treatments that are illegal and potentially harmful," the agency wrote in a news release last year. "FDA cautions consumers to make sure that any stem cell treatment they are considering has been approved by FDA or is being studied under a clinical investigation that has been submitted to and allowed to proceed by FDA."

Stat reports that as many as 200 stem cell clinics have cropped up in recent years, despite the FDA's concerns. Such clinics tout that stem cells taken from a patient's body and cultured can treat conditions from multiple sclerosis to spine injuries.

Stem cells are cells in the body that multiply to replenish dying cells. Those taken from adult patients lack embryonic stem cells' ability to morph into any kind of cell, but in the past decade adult stem cells have shown more versatility than previously thought and have become the subject of much academic research. They are considered an attractive option because, unlike embryonic stem cells, they don't pose ethical concerns nor require drugs to suppress the immune system.

But they have not been proved effective treatment for most conditions for which they are being used commercially.

The therapy gained increased attention after then-Texas Gov. Rick Perry received it for an ailing back in the summer of 2011. The following year, Perry urged companies to relocate to Texas and make the state the nation's center of stem-cell therapy.

The same year, Perry's appointees to the Texas Medical Board approved rules allowing doctors to use the therapy as long as they secured the patient's consent and the approval of a review board that evaluates clinical research for safety. It is the nation's only state policy facilitating the use of the therapy.

Perry's cells were stored by Houston company Celltex Therapeutics, then injected during surgery by a Houston orthopedist. In November, USA Today reported that Celltex cultured stem cells taken from former Dallas Cowboys running back Tony Dorsett, then injected them back to treat brain damage linked to head trauma from his playing days. The injection was done in Mexico, where Celltex moved clinical operations after being targeted by the FDA in 2013.

The FDA has scheduled a public hearing on stem cell clinic regulation for April. You can read their draft guidelines here and Stat's coverage of the issue here.