Young blood transfusion refers to transfusing blood specifically from a young person into an older one with the intention of creating a health benefit. The scientific community currently views the practice as essentially pseudoscientific, with comparisons to snake oil. There are also concerns of harm. The U.S. Food and Drug Administration, in 2019, cautioned "consumers against receiving young donor plasma infusions" stating that they are an "unproven treatment".
Experiments at Stanford University on pairs of old and young rodents placed into parabiosis suggest that the circulation of blood from young mice seems to invigorate older mice. Parabiosis experiments are difficult to generalize, as the circulatory systems of the mice are fully joined and it is unclear whether the benefits come from the sharing of blood or the older mouse's access to the younger mouse's organs. A study conducted at UC Berkeley found that blood from older mice hurt younger mice, while older mice were not benefited by the blood of younger mice.
In experiments like this, researchers found that some of these mice died quickly (11 out of 69 in one experiment) for reasons the scientists could not explain, but described as possibly some form of rejection. Amy Wagers, a researcher who coauthored several mouse studies on young blood transfusion, has said that her papers do not provide a scientific basis for some of the existing human trials.
Evidence from two large studies in 2017 showed that the transfusion of blood from younger donors to older people led to outcomes that were either no different from, or led to worse outcomes than, blood from older donors. Research on blood transfusion outcomes has been complicated by the lack of careful characterization of the transfusion products that have been used in clinical trials; studies had focused on how storage methods and duration might affect blood, but not on the differences among lots of blood themselves.