NEW YORK (AP) — Three women were left nearly or totally blind by a vision treatment at a stem cell clinic, in what doctors call a dramatic illustration of how risky such clinics can be.

Doctors who examined the women after the clinic visits said the treatment had been offered without being proven effective.

Many researchers are studying ways to treat disease with stem cells, including those taken from a patient’s own body. But authorities also warn about private clinics that offer unproven procedures.

The three women were treated for an eye condition at a Florida clinic in 2015, while they were in their 70s and 80s.

Their cases were described in Thursday’s New England Journal of Medicine.