The promise of stem cell therapy is powerful. Scientists can draw versatile cells from the human body and deliver them to repair injuries and fight disease from the inside out.

But not all stem cell therapies are created equal. Some, like bone marrow transplants, are supported by decades of clinical research and are considered safe and effective. Others are still in clinical trials, meaning they’re promising concepts but have not yet been put into practice. At the same time, medical spas and longevity clinics are increasingly marketing their own stem cell treatments for arthritis, joint pain, Parkinson’s disease, A.L.S. and even better skin.

Scientists warn that the treatments being touted by these spas and clinics are all unproven, expensive and could come with dangerous side effects.

“This clinic industry poses all kinds of risks to the public,” said Paul Knoepfler, a professor at the University of California Davis School of Medicine who studies stem cells.

Some stem cell experts, including Dr. Knoepfler, are worried that the F.D.A. may loosen oversight on stem cell therapy, which could confuse the public about the difference between scientifically supported treatment and untested therapies.

Their concerns come in part from Health Secretary Robert F. Kennedy Jr.’s remarks on the subject, including a post on X in October calling for an end to the F.D.A.’s “aggressive suppression” of stem cell therapies. During a recent podcast with longevity influencer Gary Brecka, Mr. Kennedy said that he had traveled to Antigua to get a stem cell treatment for his throat. Mr. Kennedy, who has a neurological condition that affects his throat and voice, said the therapy helped him “enormously.”