If it sounds too good to be true, it probably is.

That may be especially true when it comes to experimental treatments involving stem cells.

That’s the warning out today from researchers who looked into a purported medical trial at a Florida clinic that left three older women blind.

The researchers said there were plenty of warning signs about the experimental treatments.

These included the fact patients paid $5,000 to participate, and the trial had no control group or previous research.

The findings will be published tomorrow in The New England Journal of Medicine.

“[The paper is a] call to awareness for patients, physicians, and regulatory agencies of the risks of this kind of minimally regulated, patient-funded research,” said Dr. Jeffrey Goldberg, professor and chair of ophthalmology at the Stanford University School of Medicine, and co-author of the paper, in a press statement.

Read more: Underserved populations still lacking in clinical trials »

How women were blinded

The stem cell treatments happened in 2015.

The treatment was listed on ClinicalTrials.gov as a “study to assess the safety and effects of cells injected intravitreal in dry macular degeneration.”

That is a common, progressive disease of the retina that leads to loss of vision.

The clinical trials website is overseen by the U.S. National Library of Medicine.

Officials at the National Institutes of Health (NIH), which oversees the medical library, did not respond to Healthline’s request for an interview.

The study authors said people may have signed up thinking they were going to be part of a trial. However, the written material that accompanied the website listing did not mention a trial.

The three women who were blinded ranged in age from 72 to 88, and each suffered from macular degeneration.

Before the experimental surgery, their eyesight ranged from 20/30 to 20/200, uncorrected.

At the clinic, which was not named in the research paper, the patients had fat cells removed from their abdomens. The fat tissue was then processed with enzymes with the goal of obtaining stem cells.

Platelet-dense plasma was isolated from blood drawn from the patients. The cells were then mixed with the platelet-dense plasma and injected into the eyes. Patients reported that the entire process took less than an hour.

Soon after, the three female patients experienced side effects that included vision loss, detached retinas, and hemorrhages. They are now blind.

Dr. Thomas Albini, a study co-author, and an associate professor of clinical ophthalmology at the University of Miami, where two of the patients were subsequently treated, said the women will probably remain blind for the rest of their lives.

“Although I can't say it's impossible, it's extremely unlikely they would regain vision,” he said in a press statement.

Albini said the blindness could have been caused by the injection of a contaminant or the cell wash solution into the eye. When injected into the eye, the stem cells also could have changed into myofibroblasts, a type of cell associated with scarring.

The study authors said that even if the procedure have been properly done, there is no evidence the surgery would have improved the women’s vision.

“There is a lot of very well-founded evidence for the positive potential of stem therapy for many human diseases, but there's no excuse for not designing a trial properly and basing it on preclinical research,” Goldberg said.

Read more: Get the facts on stem cell research »

Lots of warning signs

The study authors said there were plenty of “red flags” on this particular medical offering.

The first was the lack of information on the site about a clinical trial.

There was also no hypothesis based on laboratory experiments, assignment of a control group and treatment group, collection of data, masking of clinical and patient groups, and plans for follow-up.

"There was a whole list of egregious things," said Albini.

The next warning sign was the request for patients to pay $5,000 for the treatment.

"I'm not aware of any legitimate research, at least in ophthalmology, that is patient-funded," Albini said.

Another tip-off was the fact the surgeons operated on both eyes at the same time. The study authors said professionals working in legitimate clinical trials would usually operate on one eye at a time to make sure the treatment is working properly.

“There's a lot of hope for stem cells and these types of clinics appeal to patients desperate for care who hope that stem cells are going to be the answer, but in this case these women participated in a clinical enterprise that was off-the-charts dangerous,” Albini said.

The study authors noted that listings on ClinicalTrials.gov are not fully scrutinized for scientific soundness. This particular listing is still on the site, but now states: “This study has been withdrawn prior to enrollment.”

The clinic is also no longer performing these eye injections, although it is still seeing patients, Albini said.

The procedure was also not subject to Food and Drug Administration (FDA) approval because the cells were not transferred between patients and were considered “minimally processed,” the study authors noted.

The authors recommended that people considering participation in a clinical trial check a website, such as A Closer Look at Stem Cells, for more information.

They can also investigate whether a trial is affiliated with an academic medical center.

Read more: Unproven stem cell treatments offer hope but also severe risks »