Author: Pep Guide Team

Peptides are everywhere right now: in weight-loss headlines, skincare aisles and fitness forums. But most people using the word couldn’t tell you what a peptide actually is. This guide explains it in plain English, then walks through the different kinds of peptides you’ll run into and how to judge the evidence behind them.

What is a peptide?

Amino acids are the building blocks of protein. When two or more amino acids link together through a peptide bond, you get a peptide. There’s no official cutoff, but by convention:

• Dipeptides and tripeptides have two or three amino acids (glutathione and GHK are tripeptides).
• Oligopeptides have a handful, up to about 20.
• Polypeptides are longer chains.
• Proteins are generally more than about 50 amino acids and fold into complex 3D shapes.

Size matters because it affects how a molecule behaves. Short peptides are small enough to be made in a lab by chemical synthesis. They are also usually broken down quickly by enzymes in the gut and blood, which is why most peptide drugs are injected rather than swallowed.

How do peptides work in the body?

Most biologically active peptides are signaling molecules. They travel to a target cell, bind to a specific receptor on its surface, and switch a process on or off. Think of them as short text messages between tissues.

Some examples your body already makes:

Many peptide drugs are analogs: modified copies of these natural signals, engineered to last longer or bind more strongly. Semaglutide, for example, is a GLP-1 analog built to last about a week in the body instead of a few minutes.

Peptides vs proteins vs hormones

These words overlap, which causes confusion:

• Peptide vs protein is about size. Insulin (51 amino acids) sits right on the line and gets called both.
• Hormone is about function. A hormone is any chemical messenger released into the blood. Some hormones are peptides (insulin, GLP-1). Others are steroids (testosterone, estrogen) or amines (adrenaline).

So “peptide” does not mean “natural,” “safe” or “gentle.” It just describes the structure.

The three buckets you’ll see online

1. FDA-approved peptide drugs

These have gone through phased human clinical trials and have an official label with doses, warnings and approved uses. Examples include semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound), tesamorelin (Egrifta), bremelanotide (Vyleesi), and older drugs like leuprolide and octreotide. See our GLP-1 guide for how the weight-loss drugs compare.

2. Cosmetic peptides

Ingredients like GHK-Cu (copper tripeptide-1), palmitoyl pentapeptide-4 and acetyl hexapeptide-8 show up in serums and creams. In the US they’re regulated as cosmetics, which means they can’t legally claim to treat disease. More in our GHK-Cu guide.

3. Research-use-only (RUO) peptides

Compounds like BPC-157, TB-500 and many growth hormone secretagogues are sold for laboratory research. Most of their data comes from cell or animal studies. They are not approved for human use and have no established human dose. Read what “research use only” really means before trusting any claim about them.

How to judge the evidence behind a peptide

When you read about any peptide, ask these questions in order:

1. Is it FDA-approved for anything? If yes, the label is the most reliable source for dose and safety.
2. Has it been tested in randomized, controlled human trials? Not just case reports or surveys.
3. If the data is from animals, what animal and what dose? Rodent results often fail to repeat in people.
4. Who is making the claim? A seller has a reason to overstate benefits.

Our Reading Research page goes deeper on study types and how much weight each one deserves.

Why most peptides are injected

Stomach acid and digestive enzymes are designed to chop proteins and peptides into single amino acids. That’s great for digesting food and bad for delivering an intact drug. So most peptide drugs are given as a subcutaneous (under-the-skin) injection. There are exceptions: oral semaglutide (Rybelsus) uses an absorption enhancer, and some peptides are used topically or as nasal sprays.

Our dosing policy

Pep Guide only lists a dose when an FDA label or a published human trial supports it. For everything else, we say plainly that no established human dose exists. If a website hands out precise “protocols” for a compound that has never been tested in people, that’s a sign to look elsewhere.

Scroll through almost any peptide website and you’ll see the same line: “For research use only. Not for human consumption.” Some people read that as a legal wink. It isn’t. Here’s what the research use only (RUO) label actually means, how it compares to other categories, and why it should change how you read claims online.

What “research use only” means

A research use only product is intended for laboratory work: cell studies, animal studies, analytical testing and similar research. It hasn’t been approved by the FDA as a drug, and it isn’t made, labeled or intended for use in humans.

The label describes the intended use. Under US law, intended use is judged by more than the fine print. If a seller’s website, social media or customer service suggests doses for people, describes human benefits, or sells injection kits alongside the product, regulators can treat the product as an unapproved drug no matter what the label says.

What RUO does not mean

• It isn’t a quality standard. “RUO” doesn’t mean a product was made in a sterile facility or tested for purity. Quality depends entirely on the supplier, which is why reading the COA matters.
• It isn’t a loophole. The label doesn’t make human use legal or safe.
• It isn’t proof of effectiveness. Many RUO peptides have only animal or cell data.
• It isn’t a sign of danger either. Plenty of legitimate lab reagents are RUO. The label just means the product wasn’t evaluated for human use.

RUO vs FDA-approved vs compounded

Why compounding is its own category

Compounding pharmacies mix medications for individual patients with a prescription. They can generally only use bulk ingredients that meet FDA eligibility rules, such as being part of an approved drug or appearing on specific lists. The FDA has placed several popular research peptides in a category of substances it says raise significant safety concerns for compounding. Rules here change, so check the FDA’s current bulk drug substance lists for any specific compound.

How the FDA polices the line

The FDA has issued warning letters to companies selling “research” peptides and other unapproved products while marketing them for human use. Common triggers include dosing instructions, before-and-after claims, disease treatment claims and influencer promotions describing personal use.

What this means when you read about peptides

1. Figure out the category first. Approved, compounded, cosmetic or RUO? It sets your expectations for evidence and oversight.
2. Be skeptical of human dosing for RUO compounds. By definition, there’s no official human dose.
3. Look for independent testing. For RUO products, the COA is the main quality signal you have.
4. Talk to a clinician about anything you’d put in your body. Approved options with real data often exist.

Related: What are peptides? · How to spot a sketchy vendor · Safety & Sources

Growth hormone secretagogues are compounds that prompt your pituitary gland to release more of your own growth hormone (GH), instead of supplying GH directly. The category includes an FDA-approved drug, a formerly approved drug, and a long list of research compounds. Here’s how they work and how they compare.

How the growth hormone system works

GH release is controlled by a push-and-pull system:

• GHRH (growth hormone–releasing hormone) from the hypothalamus pushes GH release.
• Somatostatin puts the brakes on.
• Ghrelin, the “hunger hormone,” also stimulates GH release through its own receptor.
• GH signals the liver to make IGF-1, which feeds back to slow further release.

Because the body’s feedback loops stay in place, secretagogues are thought to produce a more regulated GH rise than injecting GH itself. That’s the theory; long-term comparisons are limited.

Pathway 1: GHRH analogs

Tesamorelin

Tesamorelin is a stabilized version of full-length GHRH. The FDA approved it as Egrifta in 2010 to reduce excess abdominal fat in adults with HIV who have lipodystrophy. It is not approved for general weight loss or anti-aging. Its dose depends on which formulation is prescribed, so follow the current prescribing information.

Sermorelin

Sermorelin is GRF (1-29), the shortest fully active piece of GHRH. It was approved as Geref for evaluating and treating GH deficiency in children. The maker discontinued it in 2008 for business and manufacturing reasons rather than safety. It remains available in some places through compounding.

CJC-1295

A modified GRF (1-29), with or without an albumin-binding DAC group. Never approved. See CJC-1295 with DAC vs without DAC.

Pathway 2: ghrelin mimetics (GHRPs)

Ipamorelin

Developed in the late 1990s, ipamorelin was described in early research as more selective than older GHRPs, raising GH with little effect on cortisol or prolactin in animal studies. It has been tested in human trials for other purposes (such as bowel recovery after surgery) but is not approved for any use.

GHRP-2 and GHRP-6

Older synthetic GH-releasing peptides. GHRP-6 is known for strongly increasing hunger, consistent with ghrelin-receptor activity. GHRP-2 is more potent for GH but can also raise cortisol and prolactin. Neither is FDA-approved.

MK-677 (ibutamoren)

Not a peptide at all. It’s an oral small molecule that activates the ghrelin receptor. Studies in adults showed it raises GH and IGF-1 over time, along with increased appetite, fluid retention and reduced insulin sensitivity. Not FDA-approved.

Comparison table

Why the two pathways get combined

GHRH analogs and ghrelin mimetics act on different receptors. Physiology research shows that stimulating both at once produces a larger GH release than either alone. That’s the logic behind pairings like CJC-1295 with ipamorelin. Controlled long-term human data on these combinations doesn’t exist.

Side effects and risks

• Injection-site reactions, flushing and headache
• Fluid retention, joint aches, and numbness or tingling in the hands
• Increased hunger (especially GHRP-6 and MK-677)
• Reduced insulin sensitivity and higher blood sugar
• Unknown long-term effects of sustained higher IGF-1, including on existing tumors

Tesamorelin’s label lists its own warnings and contraindications, including pregnancy and disruption of the hypothalamic-pituitary axis.

Sports and drug testing

All of these compounds, including MK-677, are prohibited by WADA under the S2 category. See Peptides and sports drug testing.

Dosing

Only tesamorelin has a current FDA label with a dose, and it’s formulation-specific. The other compounds here have no established human dose, so Pep Guide doesn’t list one.

GHK-Cu is one of the few peptides you can buy in a drugstore. It shows up in serums, moisturizers and scalp products, usually as a blue-tinted liquid. It also has a long research history. Here’s what it is, what the evidence says, and how to use it without irritating your skin.

What is GHK-Cu?

GHK is a tripeptide made of glycine, histidine and lysine. It has a strong attraction to copper ions, and when bound to copper it’s called GHK-Cu. On ingredient labels you’ll often see it listed as copper tripeptide-1.

It was identified in the 1970s by researcher Loren Pickart, who found that a factor in human plasma helped older liver tissue behave more like younger tissue in the lab. That factor turned out to be GHK. Later research reported that blood levels of GHK fall substantially between young adulthood and older age.

How GHK-Cu is thought to work

Lab studies suggest GHK-Cu may:

• Stimulate collagen and elastin production by skin cells (fibroblasts)
• Support glycosaminoglycans like the ones that help skin hold water
• Influence remodeling by balancing enzymes that break down and rebuild tissue
• Deliver copper, which is required by enzymes involved in collagen cross-linking and antioxidant defense
• Have antioxidant and anti-inflammatory effects in cell and animal models

Gene-expression research has also suggested GHK affects many genes involved in tissue repair. That work is interesting but early.

What the research shows for skin

Most human skin studies were small, short and often industry-funded. The findings are encouraging, but not on the level of well-established ingredients like retinoids or sunscreen.

How people use copper peptide serums

• Patch test first on a small area for a few days.
• Start a few times a week and build up if your skin tolerates it.
• Apply to clean skin, then follow with moisturizer.
• Be careful with combinations. Many dermatologists suggest not layering copper peptides with strong acids (AHAs/BHAs) or vitamin C in the same step, since they may interfere with each other or increase irritation. Using them at different times of day is a common workaround.
• Use sunscreen daily. It does more for aging skin than any peptide.

Side effects

• Redness, stinging or irritation, especially at higher concentrations
• Breakouts in some people
• A temporary blue-green tint from the copper in some formulas
• Rarely, sensitivity or allergy to copper

Stop using it if irritation persists and talk to a dermatologist.

Topical vs injectable GHK-Cu

Topical GHK-Cu is sold as a cosmetic ingredient. Injectable GHK-Cu sold online is a research-use-only product. It isn’t FDA-approved, there’s no established human injectable dose, and quality varies by supplier. Read what RUO means and how to read a COA.

Other skincare peptides

• Palmitoyl pentapeptide-4 (Matrixyl): studied for fine lines
• Acetyl hexapeptide-8 (Argireline): marketed for expression lines
• Palmitoyl tripeptide-1 and tetrapeptide-7: often combined as a collagen-support blend

Related: What are peptides? · Research Areas

PT-141, known by its generic name bremelanotide, is one of the few peptides that went from a research compound to an FDA-approved drug. It’s also unusual because it works on desire in the brain, not blood flow. Here’s the full story, the FDA-labeled dosing, and the side effects to know about.

From tanning research to a sexual health drug

In the 1980s, researchers at the University of Arizona developed melanotan peptides to stimulate skin tanning as a possible way to reduce skin cancer risk. During testing of melanotan II, volunteers reported an unexpected effect: increased sexual arousal.

That led to the development of bremelanotide, a related peptide designed to focus on the sexual response. Early versions were studied as a nasal spray, but development moved to an under-the-skin injection, partly because of blood pressure effects seen with the nasal form.

How PT-141 works

Drugs like sildenafil (Viagra) work by increasing blood flow. Bremelanotide works differently. It activates melanocortin receptors, especially MC4R, in areas of the brain involved in sexual desire and arousal. That’s why it was developed for low desire rather than erectile function.

FDA approval: Vyleesi

In 2019 the FDA approved bremelanotide as Vyleesi for premenopausal women with acquired, generalized hypoactive sexual desire disorder (HSDD). That means low sexual desire that causes distress and isn’t caused by another medical or psychiatric condition, a medication, or problems in the relationship.

In the phase 3 RECONNECT trials, women using bremelanotide reported modest but statistically significant improvements in desire and reductions in distress compared with placebo.

FDA-labeled dosing (Vyleesi)

Follow the current prescribing information and your prescriber’s guidance.

Side effects

• Nausea: the most common side effect, most often with the first dose, and it tends to lessen with later doses
• Flushing
• Headache
• Injection-site reactions
• Temporary blood pressure increase and lower heart rate after each dose, which usually return to normal within about 12 hours
• Skin darkening (hyperpigmentation) on the face, gums or breasts in a small percentage of users, more likely with frequent use; it may not fully go away

Who should not use it

The label says not to use it with uncontrolled high blood pressure or known cardiovascular disease. It can also lower levels of oral naltrexone, which matters for people taking naltrexone for alcohol or opioid use disorder.

What about PT-141 for men?

Bremelanotide has been studied in men, including men with erectile dysfunction who didn’t respond to sildenafil. It is not FDA-approved for men. Products marketed as “PT-141” online for men are either off-label prescriptions or research-use-only products.

PT-141 vs melanotan II

Related: What are peptides? · Compound Library