Transfusions of teenage blood are being sold for £6,200 a shot, with a US company claiming it may have anti-ageing properties.
More than 100 people have undergone a clinical trial at Ambrosia, a start-up in San Francisco founded by Stanford-trained doctor Jesse Karmazin, in which they are injected with two and a half litres of plasma, the liquid portion of blood which remains after other cells are removed.
The early results have been encouraging, with Mr Karmazin telling The Sunday Times the treatment is "like plastic surgery from the inside out."
“It could help improve things such as appearance or diabetes or heart function or memory," he said. "These are all the aspects of ageing that have a common cause.
“I’m not really in the camp of saying this will provide immortality but I think it comes pretty close, essentially.”
However, researchers have warned the procedure is unproven and the trial is unlikely to provide much in the way of clinical evidence to support Mr Karmazin's claims.
It has also been criticised for failing to include a placebo group and for having participants pay to take part in the trial.
While the study is based on research suggesting old mice injected with plasma from young mice have improved memory and ability to learn, the author of that 2014 study has said there is "just no clinical evidence" the treatment would be beneficial.
"You're basically abusing people's trust and the public excitement around this," neuroscientist Tony Wyss-Coray told Science Mag last year.
In Ambrosia's trial, surplus blood is bought from blood banks, ideally from teenage donors, before the plasma is separated.
The two and a half litre transfusion, a mix from several donors, is then injected into the participants, who had a median age of 60.
Mr Kamazin claimed patients had already been seen to "look better after just one treatment."
