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But many of the products have never been extensively studied in humans, raising concerns that they could cause allergic reactions, metabolic problems and other dangerous side effects.

Here’s a closer look at the science, the hype and the potential risks surrounding the trend.

What are peptides?

Within the human body, peptides are short chains of amino acids that perform essential functions.

, for example, controls blood sugar levels and helps break down foods into energy. Likewise the popular weight loss drugs, — short for glucagon-like peptides — are based on a hormone found in the intestines that helps regulate blood sugar.

The has approved both substances as drugs. But there are many more peptides that have never been approved by regulators as safe and effective, though some have shown interesting study results in rodents and other animals.

Why are peptides so popular right now?

Synthesized peptides are not new. Some doctors have prescribed them for decades off-label, or for unapproved uses, in patients with gastric ulcers, nervous system disorders and other conditions.

In recent years, peptides have become a focus for and other public figures with large online followings. That’s driven interest in using obscure peptides for unsubstantiated uses like healing injuries, improving complexion and even extending life. Peptides in this group include an alphabet soup of injectable compounds, including BPC-157, thymosin alpha, GHK-Copper and many more. Some are banned by sports regulators as .

Experts who have studied the field are particularly concerned that some people are combining multiple peptides.

“These influencers are often advocating taking a stack of peptides each month, so it could be two, three, four different peptides,” said Dr. Eric Topol of Scripps Research Translational Institute. “This is really what I consider dangerous.”

Interest in the trend is being amplified by celebrities.

has repeatedly talked about using BPC-157 to recover from injuries. has talked about using weekly peptide injections to improve her skin and currently serves as a paid spokesperson for a company selling peptide-enriched supplements.

“If any celebrity is using a peptide, and they’re saying this is what worked for me, then of course it’s going to be more mainstream and people are going to be looking into it," said Kay Robins, a clinical nurse and operator of Pure Alchemy Wellness, a clinic outside of San Diego that sells peptide infusions and injections.

Robins says she no longer offers BPC-157 and other peptides that have been targeted by the FDA.

How are peptides regulated by the FDA?

Most of the unproven peptides promoted online are technically being sold illegally.

Any substance that is injected to produce a health benefit or prevent a medical condition is classified as a drug, which cannot be sold without FDA approval.

The agency considers many peptides to be , the most complicated and potentially high-risk type of drugs, requiring extra precautions in their manufacture and storage. In recent years, the agency has added more than two dozen peptides to a that should not be produced by pharmacies due to safety risks.

Some companies market their peptides as dietary supplements, particularly those sold as pills, gummies or powders.

While dietary supplements are less tightly regulated than drugs, the FDA still requires them to only contain ingredients found on a list of approved substances. Most peptides are not on that list and therefore are ineligible to be sold as supplements.

Experts generally agree that consuming peptides by mouth likely has little or no effect, since they will dissolve in the gut.

Who is making these peptides?

Most of the injectable peptides sold in the U.S. are produced by , which custom-mix medications that aren’t available from drug manufacturers. Pharmacies are regulated at the state level and are generally not subject to the same scrutiny as companies overseen by FDA.

In recent years, compounding pharmacies have jumped into the market for blockbuster GLP-1 drugs. Under FDA regulations, compounding pharmacies can produce their own versions of a prescription drug when there’s a shortage.

Earlier this year the FDA determined that , meaning compounders were expected to halt production. But many have of the drugs — adding extra ingredients like vitamin B, which they say benefits patients.

“There had never been the monetary incentive to push the envelope of what is legally permissible with compounding before,” said Nathaniel Lacktman, a lawyer specializing in FDA-related issues. “The dollars weren’t there.”

Some of the industry's new production capacity has gone into producing unapproved peptides, such as BPC-157.

The trend recently caught the attention of the FDA, which has added more than two dozen peptides to an interim list of substances that should not be compounded due to safety concerns.

What’s the connection between peptides and the MAHA movement?

Health Secretary Robert F. Kennedy Jr. is among those who have touted the potential benefits of peptides. He has repeatedly vowed to end “FDA’s war” on peptides, which have become popular among of his .

Some of Kennedy’s friends and are also prominent marketers of peptides, including self-described “biohacker” Gary Brecka and functional medicine physician and author Dr. Mark Hyman.

Some in the peptide field expect Kennedy to roll back FDA’s restrictions on the industry, which could include releasing a list of peptides that the agency will no longer try to keep off the market.

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AP video journalist Javier Arciga contributed to this story from San Diego

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.