Here’s a situation scientist Katrin Svensson, PhD, never expected to find herself in: Molecules she discovered in the lab and tested (so far, only in mice) have shown up on internet-based company websites for anyone to purchase and use to inject themselves.

These molecules are naturally occurring peptides, and they’re the latest wellness craze among certain sectors of the population.

But the craze is getting far ahead of the science in many ways, said Svensson, an associate professor of pathology in the Stanford School of Medicine. The peptides she’s discovered (including a new anti-obesity peptide, BRP) have not been tested in humans, a key first step between lab research, U.S. Food and Drug Administration approval and clinical use, let alone been manufactured in a regulated, safe process for humans. And while her discoveries aren’t yet the subject of viral social media reels, many other research-grade – i.e., not for-human-use-grade – peptides are.

In the name of looksmaxxing, muscle building, weight loss, or just general wellness, many people trade tips and tricks on social media and inject themselves with unapproved peptides sourced from a variety of online companies through what is known as a gray market for peptides. Is this a good idea? Definitely not, said Svensson and Jonathan Long, PhD, an associate professor of pathology who also conducts peptide research.

“First of all, there could be all sorts of contaminants in that solution that you’re injecting into yourself. Second, there’s no data about whether these peptides actually do anything or their toxicity,” Long said. “You just don’t know what you’re putting in your body.”

With an FDA advisory panel recently voting to recommend loosening restrictions for compounding pharmacies on six peptides that are far from FDA approval, the picture is even murkier. Should the FDA take the panel’s recommendation, loosened restrictions would allow patients to get certain peptides from compounding pharmacies that haven’t been through rigorous clinical testing for safety and efficacy. Long and Svensson worry that members of the public will get the wrong idea about the safety of using these peptides.

With an FDA advisory panel recently voting to recommend loosening restrictions for compounding pharmacies on six peptides that are far from FDA approval, the picture is even murkier. Should the FDA take the panel’s recommendation, loosened restrictions would allow patients to get certain peptides from compounding pharmacies that haven’t been through rigorous clinical testing for safety and efficacy. Long and Svensson worry that members of the public will get the wrong idea about the safety of using these peptides.

But there’s good news for people who are interested in the potential of peptides but are not into injecting untested and unregulated compounds: Several peptide-based drugs are FDA-approved and many more are making their way through clinical testing. The success of FDA-approved GLP-1-based weight loss drugs, which are probably the most commonly known peptide medications, have opened a door to a burgeoning new therapeutic field.

We asked Long and Svensson to break down peptide science: what these molecules are, what they do in the body, and why they might fuel the next great frontier for new and better therapies – once they are fully tested, that is.

What, exactly, are peptides?

Peptides are molecules that our body uses to send signals from cell to cell and from tissue to tissue. They’re short strings of amino acids, the same building blocks that make up proteins. Peptides are generally shorter than proteins, ranging from just three amino acids long to around 100 on the longer side. Proteins tend to be a few hundred to over 1,000 amino acids long.

Most peptides are made of proteins – the body dices up proteins into smaller chunks that are peptides – though scientists have also found some peptides that are coded directly by genes, as proteins are. Scientists have cataloged around 7,000 peptides in the human body, but it’s possible there are many more as yet undiscovered.

They act as lightning-fast messengers in our bodies, conveying signals from one organ to another, such as “just ate pasta, blood sugar incoming,” “time to sleep,” or “let’s kill this cancer cell.” For example, the GLP-1 peptide is produced in the gut when we eat a meal and travels to the brain and to the pancreas, telling these organs we’ve had enough to eat and to start releasing insulin into the bloodstream, respectively.

“Peptides can act very fast, and they can be very potent in changing our behavior,” Svensson said.

Which peptides are actually medicine?

While GLP-1 medications get all the headlines today, the first peptide-based medicine, insulin, was developed more than 100 years ago. It was discovered and isolated from a dog’s pancreas in 1921. In 1982, it also became the first commercially available synthetically manufactured peptide.

Insulin is a peptide and a hormone and is released by the pancreas to regulate blood sugar, helping shuttle glucose from the blood into cells that convert it to energy. People with type 1 diabetes are prescribed insulin because their bodies can’t make it, or make very little of it. Some people with type 2 diabetes also need insulin to keep their blood sugar steady as their bodies no longer respond correctly to their naturally produced insulin. The discovery of injectable insulin was lifesaving for diabetic patients.

Since injectable insulin came on the scene, the FDA has approved about 100 different peptide therapeutics for conditions ranging from diabetes to multiple sclerosis to septic shock.

Because peptides are broken down by the digestive system into their amino acid components, peptide drugs are nearly always injected (a few exceptions, including the recently approved oral semaglutide, a GLP-1 medication, contain very high levels of the peptide and extra engineering because so much is lost to digestion).

Until recently, people in the pharmaceutical industry assumed that patients would not want to inject themselves regularly unless it was a dire situation such as uncontrolled diabetes or growth hormone deficiency, a childhood condition that leads to stunted growth if untreated, Long said.

“Injectables were thought to be for narrow patient populations. The industry thought that you couldn’t get millions of Americans to start injecting themselves,” he said. “The fact that GLP-1s have been so successful fundamentally upends that concept.”

Over the years, scientists have discovered many peptides that serve other purposes.

“The realization that people don’t mind injecting themselves opened up the space to 100 years of chemical and scientific history related to peptides,” Long said.

What is going on with the gray market for peptides?

The broadly beneficial effects and commercial success of GLP-1 drugs also had several unintended consequences: Compounding pharmacies, which make medications on-site rather than sourcing them from drug manufacturers, have exploded in popularity as patients began seeking cheaper alternatives to the pricey commercially available GLP-1 medications. And people started exploring other peptides – many of them untested in humans – for personal use.

Online forums and social media posts abound with peptide advice: which “stacks” (combinations of different peptides) to try, where to source them, how and when to inject them. But buying and using peptides that are not from regulated sources is incredibly risky, experts say.

“I work with these experimental peptides myself. I know how potent some of them can be,” Svensson said. “I would not be comfortable taking any research-grade peptides.”

Sourcing peptides from compounding pharmacies at least avoids the potential for unknown contaminants, as these pharmacies are held to a higher regulatory standard than are research-grade peptide companies that are often headquartered outside the U.S. But that says nothing about the safety of the peptide itself.

What are the risks of unapproved peptides?

It’s possible some of these peptides will do nothing, Long said. Some peptides break down very quickly in the body, so using them could be the equivalent of injecting yourself with an expensive cocktail of amino acids. But they could also have unintended negative effects on the users’ health.

In animal studies, researchers have seen that some peptides, when used over a period of weeks or months, lead to cancer, organ damage, or neurotoxicity. Some can even aggregate into plaques in the body, mimicking the amyloid-beta plaques that are the hallmark of Alzheimer’s disease.

“They could cause all sorts of effects that are specific to that particular peptide that wouldn’t be immediately obvious,” Svensson said. “If a person takes them and they feel fine, that doesn’t mean that they are fine long term.”

Some peptides, rather than breaking down into inert substances right away could, instead, have extreme and dangerous effects, Svensson said. The animal testing of new medications that precedes clinical trials is important for figuring out safe and effective dosages. The trials ensure that dosages are safe and effective in humans as well. Many research-grade peptides available online haven’t been through that testing.

If unapproved peptides have one thing going for them, it’s that they are naturally produced by the body, meaning at least they shouldn’t provoke an immune reaction. (That doesn’t mean potential contaminants in an unregulated vial won’t trigger dangerous immune responses).

Even that may not always be the case. BPC-157 is one of two peptides commonly dubbed the “Wolverine stack” that is often touted online for speeding muscle recovery and growth after working out. BPC-157 was purportedly discovered in human gastric juice in the 1990s, but other scientists, including Svensson, have not been able to replicate the finding, leading Svensson to suspect it might not be naturally occurring, making it even riskier.

What’s next in the pipeline of peptides?

Long and Svensson, who both study the roles of different peptides in metabolism, said that the peptide medications likely to be FDA-approved the soonest are variations on GLP-1. Some, like the drug known as maridebart cafraglutide, or MariTide, combine a GLP-1 mimic with an antibody that may make it last longer in the body. Others, the experimental drugs retatrutide and cagrilintide-semaglutide, or CagriSema, combine a GLP-1 mimic with other metabolic peptides. These combinations might prove more powerful for weight loss than GLP-1 alone or could have fewer side effects.

And several clinical trials are underway to test approved GLP-1 mimic drugs for chronic conditions such as heart disease, addiction and Alzheimer’s disease.

“It will be interesting to see if we can improve some of these chronic age-related diseases with metabolic peptides,” Svensson said. “Nobody would have thought that a peptide could be a treatment for Alzheimer’s disease, and now that’s in the realm of possibility.”