What Are Peptides? Benefits, Types, Safety, and Side Effects

By: Rachel MacPherson, CPT, CSCS, PN-1

Peptides are having a moment, as you may have noticed. They're being added enmasse to skincare, and making the rounds in longevity podcasts, and gym locker rooms… basically everywhere you look. They're apparently all the rage for repairing injuries and as a shortcut for muscle building and basically fighting off death itself.

As is so often the case, the science is a bit narrower than the hype lets on. Peptides as a category actually covers a huge range of compounds with completely different jobs. A few are well studied, but plenty of others don't really have the research to back the claims.

Here is what peptides are, what they do, the types you will run into, and what the research says about safety.

What Are Peptides?

A peptide is a short chain (50 or less) of amino acids linked together in a specific order. They are actually small protein fragments, but proteins are in much longer chains. The in-between size of peptides gives them different properties than small molecule drugs and large biologics like antibodies.

Your body makes several peptides including insulin and GLP-1, which is the gut hormone that semaglutide and similar drugs mimic. Digesting protein from food also forms peptides, and they can be made for supplements or packaged foods by breaking down proteins from milk, egg, soy, or collagen.

What Do Peptides Do?

Peptides are messengers that send very specific instructions to your cells. What happens is a peptide attaches to a cell, then tells it to perform a certain task, like release a hormone, heal some tissue, or adjust blood sugar. The fact that they can be created to target very specific tasks in the body is what makes them so useful as drugs.

Almost every part of your body can be influenced by peptides, including your heart, hormones, immune system, and brain. They can help regulate blood pressure and act as antioxidants, and some food-based types might be able to lower cholesterol or help medications reach the right tissues in your body. 

For now, though, most of the evidence is based on lab tests, not on human trials, so take it all with a grain of salt until there's better evidence available.

Types of Peptides

Peptides get lumped together as a category, but there are multiple types of them. These are the most common:

• Metabolic and weight loss peptides like semaglutide and tirzepatide, which copy gut hormones to regulate appetite and blood sugar

• Recovery peptides like BPC-157 and TB-500, marketed for repairing tendons, ligaments, and the gut

• Growth hormone secretagogues like sermorelin, ipamorelin, and MK-677, which encourage release of growth hormone

• Exercise mimetics like MOTS-c, which copy some of the cellular signals you get from training

• Food derived bioactive peptides from collagen, whey, milk, and soy, found in supplements and functional foods

Only the weight loss and metabolic peptides really have enough good human research and medical approval for conditions like diabetes, hormone therapy, and age-related conditions. Other types are promising but don't really have enough proof yet.

Benefits of Peptides: Do Peptides Really Work?

Several peptides are approved and are used for treating metabolic disease, hormone conditions, and infections. These are the ones that had trials to determine the ideal dosing and potential side effects.

Other peptides like the ones for injury repair don't have a very solid body of evidence on their side. What data there is comes from animal and cell studies and might look exciting, but there are little to no human trials, so it's not possible to say what an effective (or safe) dose is or what the risks could be.

There also aren't any peptides that are going to reliably build muscle (bummer, we know). Growth hormone secretagogues might support your gains a bit if you've already got your training, sleep, and nutrition habits pretty near perfect, but that's not the case for most people. If you actually want to build muscle, you still need to do it the old school way with enough protein, creatine, and progressive overload.

Are Peptides Safe? Side Effects to Know

Peptides can be safe, but they also might not be, it totally depends on what you're taking. Peptides that are approved for medicinal use are typically safe, but other types haven't been held to the same standard, so it's taken at your own risk, depending on which peptide it is. Collagen peptides, for example, have a lot of research on their safety and effectiveness, but some of the online gym bro lab ones could do damage.

Side effects can be an allergic reaction or immune issues, and some kinds might be contaminated or include the wrong dose. They also might not be all that shelf stable or be poorly processed. Many peptides (especially ones bought on the gray market), don't have long term human safety data either. Because these substances can be toxic, you should always consult a licensed doctor before using any non-approved peptide.

How to Take Peptides

Most peptides won't make it intact through your digestive tract and will be broken apart, so you could end up paying for something that never reaches your bloodstream (which means it won't work). This is the reason prescription peptides are almost always injected under the skin, or there are a few nasal and topical ones as well.

It's also important to store peptides so they don't degrade. Keep them away from heat and light and also beware that shaking and mixing them with water can degrade them as well.

The best bet is to go through a doctor and treat peptides as an addition to the basics instead of some sort of hack or replacement for actual meds. Basic habits like getting your ideal amount of protein and calories in each week will make a much bigger difference.

Should You Take Peptides?

The risk to reward definitely depends on which peptide you want to take. Very few are well studied and genuinely effective, while most of the trendy ones are still waiting on human trials to back them.

Talk to a clinician before trying anything that isn't approved. If you train consistently and get enough protein, adding a tried and true basic like creatine will outperform pretty much anything in a vial.

Rachel MacPherson is a Certified Strength and Conditioning Specialist, Certified Personal Trainer, Nutrition Coach, and health writer with over a decade of experience helping people build strength and confidence.

This article was reviewed by Rosie Borchert, NASM-CPT, for accuracy.