Promising stem-cell research in eye disease and other conditions is taking place. But researchers and health officials have been warning for years that patients are at risk from hundreds of private clinics that have sprung up around the United States and overseas, offering stem-cell treatments for all manner of ailments, like injured knees, damaged spinal discs, neurological diseases and heart failure. Businesses promising “regenerative medicine” have multiplied, with little or no regulation.

Stem cells, which can develop into many different types of cells, are thought to have tremendous potential to repair or replace tissue damaged by disease, injury or aging. But so far, the F.D.A. has approved only a few stem-cell products to treat certain blood disorders.

The women in Florida suffered detached retinas, in which the thin layer of light-sensing cells that send signals to the optic nerve pulls away from the back of the eye — a condition that usually needs prompt surgery to prevent blindness. Doctors who examined the patients said they suspected that the stem cells had grown onto the retina and then contracted, pulling it off the eyeball.

One woman had such high pressure inside her eyes — about three times the normal level — that it may have damaged her optic nerves. Doctors operated quickly to relieve the pressure, but she became blind.

“The really horrible thing about this is that you would never, nobody practicing good medicine would ever do an experimental procedure on a patient on both eyes on the same day,” said Dr. Thomas A. Albini, an author of the article who saw two of the patients, at the Bascom Palmer Eye Institute at the University of Miami Miller School of Medicine.

Standard practice, he said, is to treat one eye at a time, usually the worse eye first, so that if something goes wrong at least the patient still has one eye left with some vision.