Injectable peptides that people use to build muscle, shed weight and appear younger, are on the rise in the U.S. and trending among influencers, celebrities and fitness coaches, despite the fact that they aren't approved by the Food and Drug Administration (FDA). While they can be bought online for between $300 and $600 a vial, the products have not been extensively trialed or studied in humans, raising concerns among experts that they could pose certain unknown health risks. Peptides exist in