The Food and Drug Administration is discouraging people from being infused with young blood, a procedure that's becoming increasingly common but hasn't been proven to have medical benefits.

The procedure works like it sounds. People are infused with blood plasma donated from young people. Some clinics claim the procedure works like a fountain of youth to reverse aging and memory loss or even treat diseases like dementia, Parkinson's disease, multiple sclerosis, Alzheimer's, heart disease or post-traumatic stress disorder.

Companies typically charge thousands of dollars for the treatments. That concerns the FDA, which says these infusions are "unproven and not guided by evidence from adequate and well-controlled trials."

"Simply infusing plasma is not a benign intervention and should not be used in such cavalier fashion," FDA Commissioner Scott Gottlieb told CNBC in a phone interview Tuesday.