But there is no clear evidence that these treatments work, and their safety has yet to be established. Most researchers, including those at the National Institutes of Health, think that efforts to sell therapies involving adult stem cells, which can develop into different types of cells to replenish tissue, have gotten way ahead of the science.

Even so, hundreds of clinics have popped up around the country to meet the demand. Some of the clinics also inject joints with platelet-rich plasma, a solution of platelets extracted from the patient’s own blood. A few employers have even agreed to provide insurance coverage for the treatments.

There is almost no regulatory oversight of orthopedic procedures using bone-marrow extracts or platelets, which are regarded as low risk. While the Food and Drug Administration insists that it does have the authority to regulate stem cell treatments, it adopted an industry-friendly approach in 2017 by giving companies a three-year grace period in which to describe their products or treatments so the agency can determine whether they meet the criteria of drugs that would require agency approval. So far, few companies have submitted any information.

In the meantime, rogue clinics offering other kinds of procedures have flourished, accused of blinding people by injecting cells into their eyes, mixing stem cells with smallpox vaccine to treat cancer or causing severe infections by administering contaminated blood from umbilical cords into patients’ joints or spines. In some of the worst cases, patients had already been harmed before the agency took any action, and the patients took legal steps themselves, suing the clinics that injured them.

“We had our day job clearly cut out for us,” said Dr. Scott Gottlieb, who was the F.D.A. commissioner until April. There is, he added, a lot of “really bad stuff.”