The phenomenon of stem cell tourism has been associated with travel to exotic locations such as China, Argentina or Mexico, where commercial clinics with little accountability offer high hopes, expectations and — sometimes — the promise of miracle treatments for diseases ranging from muscular dystrophy to spinal cord injury.

However, a study out June 30 suggests that “stem cell tourists” from the United States need not leave the country to find these treatments. Such clinics are becoming more common in the U.S. —  with California  leading the way.

The study, published in the journal Cell Stem Cell, showed there are at least 351 businesses operating 570 such clinics nationwide, many of them claiming to treat a wide range of illnesses with services that lack  scientific evidence or oversight.

California has 113 of those clinics — nearly one in five, according to the study. And Southern California hosts some of the “hotspot” cities, including Beverly Hills, with 18 clinics, and Los Angeles with 12.  Florida has the second highest number after California, with 104 clinics, followed by Texas with 71 and Colorado with 37.

“There are businesses operating in the United States that are making marketing claims that are just as problematic as businesses in other countries,” said Leigh Turner, coauthor of the study and a bioethicist at the University of Minnesota.

Stem cells, also known as master cells, are specialized cells in the human body that can multiply and evolve with remarkable potential for healing. For example, blood stem cells have been used for decades with the Food and Drug Administration’s approval to treat leukemia and several blood disorders. More recently, some high-profile experimental therapies have been credited with healing well-known sports figures such as Peyton Manning and Bartolo Colon. But much of this science and understanding is still taking shape, with stem cells sometimes proving difficult to control, and in some instances causing tumors and other adverse outcomes.

For example, the New England Journal of Medicine and The New York Times chronicled the story of a 66-year-old man who sought stem cell treatments abroad for the effects of a stroke. But instead of a reversal of his symptoms, he started losing his ability to walk and had to seek for medical help. The doctors were startled to find a bloody mass of tissue growing in his spinal cord.

Turner’s coauthor, Paul Knoepfler, a professor at UC Davis School of Medicine, started this investigation when he noted an uptick in the frequency of people inquiring online about these clinics in the U.S. rather than abroad. The researchers collected information by searching the internet for keywords such as “stem cell treatment” and analyzing the companies’ websites. The search covers the time frame between Sept. 1, 2015 and Feb. 29, 2016.

“They make strong marketing claims offering stem cell treatments for things like ALS, muscular dystrophy, Parkinson’s disease, Alzheimer’s … that’s where there are pretty profound questions on whether they are compliant with the FDA regulations,” Turner said.

Through their searches, Turner and Knoepfler found great variation in these clinics. The treatments they offered ranged from anti-aging and other cosmetic applications to therapies for diabetes, cardiac and ophthalmological diseases as well as Parkinson’s and other degenerative conditions. Most clinics advertised stem cell treatments for orthopedic uses, followed by pain relief and sports injuries. Thirty-three clinics made marketing claims regarding muscular dystrophy and nine clinics each promoted stem cell treatments for autism and cerebral palsy, targeting parents and family members of patients.

The prevalence of these advertisements raises ethical issues because most claims lack peer-reviewed scientific evidence that the treatment is safe or even effective, noted the authors. The FDA has issued a warning about some of the claims. Meanwhile, the International Society for Stem Cell Research released guidelines last month that warned the majority of treatments these clinics offer are unproven, with “adverse events” frequently reported after procedures.

“When you’re seeing a company that is making claims that it is offering stem cell treatments for 30 or 40 disorders, there are good reasons to think that that is not compliant,” Turner said. “It’s kind of mystifying why the FDA hasn’t taken a closer look at these businesses.”

The FDA is, in fact, cracking down on stem cell clinics, with multiple warning letters given and draft and final guidelines issued. But Turner is concerned that, without strong and consistent regulation and enforcement, more businesses will crop up.

Meanwhile, stem cells are being explored as treatment options for ailments ranging from stroke and heart diseases to bone fractures and many others, but the vast majority of these treatments are still in clinical trials.

“There are very few stem cell therapies that are ready for clinical application,” said Timothy Caulfield, research director of the Health Law Institute at the University of Alberta in Canada and member of the stem cell society’s guideline task force. “It is often portrayed [by clinics] as if there are some kind of regulatory hurdles keeping these treatments from getting to [them]. That is not the case. The case is that the science for most cases is not there yet.” Caulfield was not affiliated with the study.

Though news coverage of stem cell treatments gone bad has been considerable, there have also been documented successes. Turner is worried that poor results from unregulated stem cell clinics might undermine the credible, peer-reviewed research that is underway.

“A lot of businesses are making claims with no substantial evidence behind it,” he said. “People should try to educate themselves and not be preyed upon.”

This story was produced by Kaiser Health News, an editorially independent program of the Kaiser Family Foundation.