Matt Kaeberlein, a longevity researcher, first started hearing about peptides a few years ago. “At that point it was mostly functional medicine doctors that were using peptides,” he says, referring to physicians who embrace alternative medicine and supplements. “In the last six months, it’s kind of gone crazy.” Peptides have gone mainstream. At the health-technology startup Superpower in Los Angeles, employees can get free peptide shots on Fridays. At a health food store in Phoenix, a sidewalk sign reads, “We have peptides!” At a tae kwon do center in South Carolina, a peptide wholesaler hosts an informational evening. On social media, they’re everywhere. And that popularity seems poised to grow; Department of Health and Human Services secretary Robert F. Kennedy Jr. has promised to end the FDA’s “aggressive suppression” of peptides. The benefits and risks of many of these compounds, however, are largely unknown. Some of the most popular peptides have never been tested in human trials. They are sold for research purposes, not human consumption. Some are illegal knockoffs of wildly successful weight-loss medicines. …