“There’s this new kind of wedge in between supplements and prescription drugs that are called peptides,” Dr Andrew Huberman, a neuroscientist from Stanford (with a cult following of longevity seekers), said recently on the Flagrant podcast. “I’ll tell you right now, a lot of the bodies you see in Hollywood films are on peptides.”

But the word peptide is actually quite vague and meaningless when it comes to describing a health intervention. The truth is we all have peptides inside our bodies right now – they’re just chains of molecules (specifically, amino acids) that we produce naturally.

Here’s what you should know.

Why do people use peptides?

Peptides are big in anti-ageing circles. Business Insider recently reported on an 86-year-old who says she’s “physically immortal” – she is doing peptides. So is billionaire longevity investor Christian Angermayer. Many biohackers claim peptides can improve brain power and joint health.

In the fitness world, some athletes swear by certain peptides for faster recovery from workouts and better stamina.

Those specialising in men’s health are using peptides to “restore” or “balance” hormones, specifically testosterone, or to boost libido.

And the bodybuilding community has embraced peptides as a tool to boost muscle growth and burn fat faster, often used with various supplements in combinations known as “stacks”.

Some bodybuilders have also embraced peptides as a more natural alternative to steroids – which are also, notably, illegal to use without a prescription, although their common use is something of an open secret among the community.

How do you do peptides?

Most likely, when people say they’re “doing peptides”, they mean they are popping a pill or injecting a drug in the hopes of improving ageing or building muscle.

But when people talk about peptides for anti-ageing and performance, they’re usually talking about growth-hormone mimicking peptides.

“Certain peptides, for example, will make your body naturally produce more human growth hormones again,” Angermayer, the longevity investor, said. “Indeed, biohackers are very excited about it because you can practically make your body do certain things.”

High demand has sparked a black market for peptides

Sermorelin is a particular peptide. It prompts the body to release more growth hormone, which can help for building muscle mass or improving their libido.

However, while sermorelin is available as a prescription drug, the high demand has led to shortages – which has sparked a whole shadow industry of unregulated sermorelin peddlers on the internet.

Many peptides are classified as biologics, which means there are strict laws around who can make and sell them.

In some cases, users are even sourcing peptides online from sites that market “research chemicals”. While the vials are explicitly labelled not for human use, the sites are full of coded language hinting at “fast-acting, premium research gains” or products promising a “monster pump” or superior mass.

The risks of taking peptides incorrectly

While peptides are generally perceived as low risk since many are based on substances that naturally occur in the human body, the dose matters – and so do any potential additives or contaminants.

Some common peptides sold as fitness supplements, like creatine, have extensive research supporting the benefits and general safety in human use. However, other popular peptides are in early stages of study, often on mice instead of people.

As with all supplements, a lack of comprehensive regulation can make it difficult to know what you’re getting, and how much. It’s important to take the peptide hype with a grain (or even a whole shaker) of salt until the evidence catches up with what biohackers and influencers have claimed.