What Are Peptides? A Plain-English Guide

What are peptides? They are short chains of amino acids: the same small building blocks that form proteins. A common scientific convention puts peptides at 2 to 50 amino acids and proteins above that range, although the boundary is not absolute. Your body makes peptide signals including insulin, GLP-1, oxytocin and growth-hormone-releasing hormone. Medicines can copy or modify those signals. The practical point is simple: peptide describes a kind of molecule, not one treatment, so evidence, risks and regulatory status must be judged compound by compound.

What are peptides made of?

Amino acids join through peptide bonds to make a chain. The sequence matters: rearranging the amino acids can change the chain's shape, the receptor it recognizes and what happens after that receptor receives the signal. A 2023 Nature Reviews Methods Primers overview describes peptides as biopolymers typically containing 2 to 50 amino acids and notes their roles as hormones and other signaling molecules.

The cutoff is a convention, not a border enforced by chemistry. Small proteins and large peptides overlap around it. What usually distinguishes a peptide in practice is a relatively short amino-acid sequence with a specific biological message. Oxytocin is only nine amino acids long. BPC-157 has 15. Insulin sits just beyond the usual cutoff but is still routinely discussed as a peptide hormone.

Length identifies the molecular class; it does not establish medical value. BPC-157's short sequence makes it a peptide, but its evidence base remains largely preclinical and should not be treated as equivalent to the evidence behind an established peptide drug.

The peptide categories a prescriber considers

The word covers very different compounds. A useful clinical map starts with the signal or system involved, then asks what human evidence exists for the individual molecule.

These labels are organizational, not promises of benefit. Two peptides in the same row can differ in receptor target, route, evidence, side effects and regulatory status. That is why a prescriber evaluates the exact compound rather than recommending “peptides” as one interchangeable therapy.

How peptide signals reach a cell

Many signaling peptides act like brief messages between tissues. A cell releases the peptide; it travels locally or through the bloodstream; then its shape fits a receptor on another cell. Receptor activation starts a chain of events inside that cell. Insulin signals tissues to handle glucose, GLP-1 participates in post-meal metabolic signaling, oxytocin helps coordinate labor and milk release, and GHRH signals the pituitary to release growth hormone.

That is the mechanism in outline. The receptor-by-receptor detail belongs in the peptide mechanism library, because there is no single action shared by every peptide.

Why most peptide medicines are injected

The digestive system is designed to break dietary proteins and peptides into smaller pieces. Enzymes can cut peptide bonds, and an intact peptide often crosses the intestinal wall poorly. A 2013 Clinical Pharmacokinetics review describes both problems—proteolytic breakdown and low permeability—as reasons therapeutic peptides usually have limited oral bioavailability. Injection bypasses much of that barrier and delivers a more predictable amount.

Oral semaglutide is the useful exception because it shows how much engineering an oral peptide may require. Its tablet combines semaglutide with the absorption enhancer SNAC. In a 2018 Science Translational Medicine study, absorption occurred in the stomach near the tablet, while SNAC locally buffered the environment, protected semaglutide from enzymatic degradation and temporarily increased absorption. The oral semaglutide explainer covers why that tablet cannot be generalized to ordinary peptide capsules.

Route still depends on the molecule and formulation. Some peptides are used topically or through the nose, and a few have engineered oral delivery. “Peptide” does not automatically mean “injection,” but injection remains common for a sound pharmacology reason.

Prescription peptides versus grey-market vials

A molecule's name does not tell you who made the finished product, whether a clinician reviewed the request or whether a pharmacy dispensed it for an identified patient. Those process facts are the legitimacy line.

At Promise, a licensed provider reviews every request and not everyone qualifies. If a prescription is written, a licensed U.S. compounding pharmacy prepares and dispenses the formulation on that provider's order. Grey-market vials sold as laboratory material bypass that patient-specific prescribing and dispensing chain. The difference is accountability for the finished product, not a claim that every prescribed peptide has equally strong clinical evidence.

As of August 2026, FDA's compounding Q&A states that federal law addresses compounding by licensed pharmacists in state-licensed pharmacies, physicians and registered outsourcing facilities. It also explains that the agency does not review compounded drugs for safety, effectiveness or quality before marketing.

Approved peptide drugs do exist, including insulin and branded medicines containing semaglutide or tirzepatide. A peptide dispensed through Promise is a compounded medication, which is different from an FDA-approved product: the formulation offered here is not FDA-approved. Regulatory status is one input into care; a licensed provider may still prescribe a compounded formulation when appropriate, and that decision belongs to the patient and the doctor.

A growth-hormone-signaling option such as sermorelin still needs its own clinical assessment. Sharing the peptide label with semaglutide does not make their indications, evidence or risks interchangeable. For a compound-level risk framework, start with how peptide safety varies.

Three questions that clarify any peptide claim

First, what is the exact molecule? A broad category cannot answer a compound-specific question. Second, what kind of evidence supports the proposed use—human trials, observational data or only animal and laboratory work? Third, who is accountable for prescribing, dispensing and follow-up?

Those questions separate understandable biology from a medical decision. The prescription process for peptides explains what happens from a compound-specific intake through provider review, pharmacy dispensing and follow-up.