Definition
A peptide is a short chain of amino acids — typically between two and fifty — linked by peptide bonds. Below that length, you have a free amino acid. Above it, you have a protein. Peptides sit in the middle: large enough to fold into a recognizable shape, small enough to cross tissue barriers, and just specific enough to bind a single class of receptor with high affinity.
The distinction matters because mechanism drives effect. A peptide is not a vitamin, not a hormone analog by accident, not a small-molecule drug. It is a sequence — and that sequence either matches a binding site or it does not.
How peptides signal
Most therapeutic peptides act as ligands. They dock with a membrane-bound receptor on a target cell, triggering a conformational change that cascades through second messengers — cyclic AMP, calcium, MAP kinase — and ultimately changes gene expression or protein behavior inside the cell.
A few peptides act intracellularly. Some, like BPC-157, appear to modulate growth-factor pathways without binding a single canonical receptor. Others, like NAD+ precursors, are technically nucleotides that participate in redox chemistry rather than receptor binding. The category 'peptide therapy' is loose; the mechanisms are not.
Why route of administration matters
Peptides are fragile. Oral bioavailability is usually under five percent because stomach acid and intestinal proteases break peptide bonds before absorption. That is why most therapeutic peptides are injected subcutaneously or, in select cases, delivered intranasally or transdermally.
Subcutaneous injection gives predictable absorption: peak plasma concentration in fifteen to ninety minutes, half-life from minutes to days depending on the molecule. The longer half-lives — semaglutide at one week, for instance — come from engineered modifications that resist enzymatic clearance.
| Route | Bioavailability | Onset | Practical |
|---|---|---|---|
| Oral | <5% | Variable | Rare; specialty formulations only |
| Subcutaneous | 70-95% | 15-90 min | Standard for most therapy peptides |
| Intramuscular | 80-95% | 30-60 min | Used for select longer-acting agents |
| Intranasal | 10-40% | 10-30 min | Convenient; lower exposure |
| Transdermal | <10% | 60+ min | Limited to small peptides |
Therapeutic categories
Modern therapeutic peptides fall into a handful of working buckets: metabolic (GLP-1, GIP, amylin); growth-hormone axis (CJC-1295, ipamorelin, sermorelin, tesamorelin); tissue repair (BPC-157, TB-500); cognitive (selank, semax, dihexa); skin and pigment (GHK-Cu, melanotan); and longevity (epitalon, NAD+ precursors, MOTS-c).
Each category has its own mechanism, its own evidence base, and its own safety profile. Treating them as one undifferentiated category — the way most consumer-facing peptide marketing does — is how patients get hurt.
Bottom line
A peptide is a signal. The right peptide, at the right dose, delivered the right way, can move a specific biological lever with remarkable precision. The wrong peptide — or the right peptide misused — does nothing, or worse.
The rest of this journal exists to help you tell the difference.
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