Health Secretary Robert F. Kennedy Jr., who has built a national following among Americans who are deeply skeptical of health experts, pharmaceutical companies and traditional medicine. Angelina Katsanis/Politico/AP

More Americans are injecting themselves with unapproved chemicals that are pitched as ways to build muscle, rejuvenate skin and extend life, the latest example of the nation’s fascination with alternative therapies and wellness hacks .

Behind the trend is the surging popularity of GLP-1 weight loss medications , a class of so-called peptides approved to help users quickly shed pounds .

But the peptides being promoted by influencers , celebrities and wellness gurus are different: Many have never been approved for human use and much of their purported evidence c

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