The latest advancement in anti-aging therapies hardly sounds like modern medicine at all. Ambrosia, a startup based in Monterey, California, is launching a clinical trial to inject the blood of young people into just about anyone aged 35 and up—if they’re willing to pay $8,000.

Young-to-old blood transfusions hit the limelight in 2014 when Harvard Stem Cell Institute researcher Amy Wagers discovered that the blood of young mice improved muscle, heart, and brain function in older mice. Scientists sewed the skin of two mice together, allowing their circulatory systems to merge. The surgery, called parabiosis, was first performed in the 1860s but has seen a resurgence of interest following Wagers’s study.

PayPal cofounder and Facebook investor Peter Thiel recently told Inc. that he finds parabiosis “really interesting” and said that his curiosity leans more toward his personal health rather than a potential business venture. This is not the first time Thiel has shown an interest in combatting aging.

Peter Thiel

Ambrosia is planning on giving 600 patients four rounds of weekly blood infusions coming from 16- to 25-year-olds. Ambrosia founder Jesse Karmazin, who went to Stanford Medical School, said in an e-mail that he was inundated with interview requests and unable to speak today.

The pay-to-play nature of Ambrosia’s trial has draw criticism from some researchers, according to Science, and is reminiscent of recent news highlighting the surge in unproven stem-cell clinics that require their patients to pay for clinical trials that may never see data published. The Ambrosia clinical trial lists over 100 blood biomarkers it plans to measure before and one month after the transfusions, but David Glass, executive director of aging research at Novartis, says that because the trial has no placebo control group, it will be impossible to decipher any benefits from it.

Another company called Alkahest has been conducting a clinical trial with young blood infused into patients with probable Alzheimer’s. Its results are expected out by the end of the year.

In 2014, Wagers published two studies in Science outlining the role of GDF11, a protein growth factor in the blood that she said was responsible for the benefits of parabiosis. One study showed that GDF11 improved muscle strength in old mice, and another demonstrated its restorative effects on the aging brain.

Wager’s research suggests that GDF11 levels decline with age, and that restoring it can have dramatic effects.

But several studies have cast doubt on the role of GDF11 in anti-aging processes. Scientists at Novartis published data contradicting Wagers’s study, showing that GDF11 actually increases with age and is counterproductive for regenerating skeletal muscle. Wagers responded in kind with a follow-up study reaffirming her previous findings that levels of GDF11, along with a closely related protein known as GDF8, both decrease with age.

But the last word is not out. A study conducted by scientists at GlaxoSmithKline and Five Prime Therapeutics found that GDF11 wasn’t involved in rejuvenating aging muscle cells. And in June, scientists found that providing GDF11 therapeutically doesn’t improve muscle function in mice with Duchenne muscular dystrophy.

Wagers maintains that the roles of GDF11 and its cousin protein need to be sorted out. The case isn’t closed, but even if GDF11 alone isn’t the potent anti-aging protein it was first thought to be, Wagers’s studies still suggest that there is something in young blood promoting rejuvenation.

(Read more: “Can Compounds in Young Blood Fix Aging?,” “A Contrarian in Biotech,” “Clinics Offering Unproven Stem Cell Therapies Are Proliferating Across the U.S.”)