What are peptides?

The short version

Proteins are built from small molecules called amino acids, the building blocks of protein. Join a few amino acids into a short chain and you have a peptide. Keep adding links and, eventually, the chain is long enough to be called a protein.

Your body makes many peptides on its own. Insulin, the hormone that helps your body use the sugar from food, is one of them.

Some peptides are prescription medicines. Others are sold online with a label that says "research use only", which means the seller is not offering them as medicine for people. The word "peptide" on its own tells you nothing about which kind you are looking at, or whether it is safe or legal to use.

A peptide, piece by piece

Picture amino acids as beads and a peptide as a short string of them. Two things give a peptide its identity: which beads are on the string, and the order they come in. Swap a single bead and you have a different molecule, often with a different name.

Five peptides beginners often ask about show how different the chains can be:

• GHK-Cu is just three amino acids long, holding a single copper atom. The same three-link sequence, GHK, also sits inside collagen, the protein that gives skin and connective tissue their structure.
• BPC-157 is a synthetic chain of fifteen amino acids, modelled on part of a protein found in human stomach juice.
• Sermorelin is twenty-nine amino acids long. It copies the first twenty-nine links of a hormone the brain makes, called growth hormone-releasing hormone.
• Semaglutide is thirty-one amino acids long. It is modelled on a gut hormone called GLP-1 (short for glucagon-like peptide-1), with a few links changed on purpose and a fatty side chain attached.
• Tirzepatide is a synthetic chain of thirty-nine amino acids, designed to act on the receptors for two gut hormones, GIP and GLP-1, at once. A receptor is the docking point on a cell that a hormone fits into.

One name often listed beside these, NAD+ (nicotinamide adenine dinucleotide), is not a peptide at all. It is a coenzyme: a helper molecule every cell uses in the chemical reactions that handle energy.

Peptides your body already makes

Your body was making peptides long before anyone put one in a vial. Many hormones, the chemical messengers that carry signals around the body, are peptides.

• Insulin is a peptide hormone, and one of the best known.
• GLP-1 is made in the gut. Semaglutide and tirzepatide were both designed around it.
• Growth hormone-releasing hormone is made in a part of the brain called the hypothalamus. It sends its signal to the pituitary gland, which sits just below the brain. Sermorelin copies part of this hormone.

So when people ask what peptides do, the honest first answer is that, inside the body, many of them carry messages. That describes natural chemistry. It is not a promise about what any product will do for you, and Neo does not make those promises.

Three very different things can be called a peptide

Here is where beginners get caught out. The same word covers three very different things.

1. Medicines the FDA has approved

The Food and Drug Administration (FDA) is the U.S. agency that reviews medicines before they can be sold. When it approves a drug, it has looked at the evidence for that specific product and agreed that it works and is safe enough for a named use. Semaglutide and tirzepatide are the active ingredients in FDA-approved medicines, and several other peptides are approved for particular conditions. Sermorelin was once approved too, but that product was later discontinued.

2. Medicines a pharmacy makes to order

Compounding means a pharmacist mixes or makes a medicine for one patient, working from a prescription, instead of handing over a ready-made product from a drug company. Compounded medicines are real medicines, and they can be legal. They are not FDA-approved: the FDA does not check that a compounded medicine works, that it is safe, or that it was made well before it is dispensed. Compounded semaglutide and tirzepatide belong in this group. They are not FDA-approved, and FDA-approved versions of both exist.

3. Products labelled "research use only"

The third group is sold online in vials with a label such as "for research use only" or "not for human consumption". These are not medicines. They are not a lawful route to treatment: when they are sold for people to use, the FDA treats the sale as illegal. No clinician is involved in the sale.

Why "research use only" is a warning, not a loophole

A research-only label can sound like a technical detail. It really tells you that the seller is not taking responsibility for the product as a medicine for people.

Under U.S. law, what makes something a drug is what it is sold for, not the wording on its label. If a website talks about weight, healing or hormones, and sells mixing water, a dosing chart or a calculator next to the vial, the FDA reads that as a product meant for people to inject. It has said so in writing, and it has sent warning letters to sellers who did exactly that.

That matters for three plain reasons:

• Nobody reviews your health. There is no prescriber to ask about your other medicines, your conditions, or a possible pregnancy.
• No one is answerable to you for what is in the vial, and it is hard to be sure what it contains. The FDA's usual worry about injected peptides is that the body may react to them.
• An injection skips your body's first defences. Anything injected goes straight past the barriers that would normally catch germs. That is why making injectable medicines comes with strict sterile-manufacturing rules.

No seller needs naming here. The label itself is the warning.

The same molecule can live under different rules

The same peptide can be a cosmetic in one product and a drug in another. GHK-Cu appears in face creams, where it is treated as a cosmetic ingredient; in a syringe, the same molecule is judged as a drug. What decides it is what the product is sold to do.

What this page leaves out, on purpose

This page explains what peptides are. It does not say what any of them will do for you, and it gives no doses. Those answers depend on your own health, and a website cannot examine you. That is a conversation for a licensed clinician who has seen your history. If you already have a goal in mind, the goals page explains the ten goal areas in plain words.

Talking to a licensed clinician

Whether a prescription peptide could fit you is a question for a licensed clinician. Promise Peptides is a U.S. telehealth platform. Promise prescribes peptides now through licensed U.S. providers. A licensed U.S. provider reviews every request and your health history, then decides whether to prescribe. Not everyone qualifies, and the answer can be no. Prescribed medicines are prepared by licensed U.S. compounding pharmacies. Each pharmacy separately decides whether it can lawfully prepare a prescription; a prescription does not guarantee it will be filled. Compounded medicines are not FDA-approved. U.S. patients only. Availability varies by state and by treatment. Promise is LegitScript-certified. That certification is not FDA approval.

Visit Promise Peptides for energy

Neo may earn money when readers become Promise patients.

Sources

1. Compounding and the FDA: Questions and AnswersU.S. Food and Drug Administration
2. Understanding the Risks of Compounded DrugsU.S. Food and Drug Administration
3. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight LossU.S. Food and Drug Administration
4. Drugs@FDA: FDA-Approved DrugsU.S. Food and Drug Administration
5. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety RisksU.S. Food and Drug Administration
6. 21 U.S.C. 321, Definitions (the legal meaning of "drug")Office of the Law Revision Counsel, U.S. House of Representatives
7. 21 CFR 201.128, Meaning of "intended uses"eCFR, National Archives
8. FDA Authority Over Cosmetics: How Cosmetics Are Not FDA-Approved, but Are FDA-RegulatedU.S. Food and Drug Administration
9. Clinical Pharmacokinetics of Semaglutide: A Systematic Review (Drug Des Devel Ther, 2024)PubMed
10. LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: from discovery to clinical proof of concept (Mol Metab, 2018)PubMed
11. BPC-157 and inflammatory bowel disease research: 2006 paper in Inflammopharmacology (descriptive label)PubMed
12. Growth hormone-releasing hormone and its analogues in health and disease (Nat Rev Endocrinol, 2025)PubMed
13. NAD+ metabolism and its roles in cellular processes during ageing (Nat Rev Mol Cell Biol, 2021)PubMed