Three women are completely blind after undergoing unproven stem cell treatment in a clinical trial that went badly wrong.

The patients aged from 72 to 88, all suffered from macular degeneration which leads to vision loss, and were paid $5,000 to undergo the procedure.

But within a week of starting the "off-the-charts dangerous" therapy at an American clinic, the patients suffered complications.

They suffered from further vision loss, detached retinas, bleeding and are now totally blind and unlikely to recover.

A medical journal which reported the story has prompted warnings of the risks to patients taking part in experimental stem cell therapy studies.

The procedure failed to meet even the most basic requirements of a clinical trial and was not even founded on science, experts say.

"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," said Dr Thomas Albini, from the University of Miami, who co-authored the study published in The New England Journal of Medicine.

"It's extremely unlikely that they would regain vision."

He continued: "This definitely shows that the lack of oversight can lead to bad players and bad outcomes. It's alarming."

The therapy involved treating fat tissue from the abdomen with enzymes to obtain "adiopose-derived" stem cells, which were mixed with blood plasma and injected into the women's eyes.

Unusually, both eyes were treated at once - most experimental eye procedures are tested on one eye first so if something goes catastrophically wrong, the patient can still see with one eye.