What Are Peptides? A Beginner's Guide to Peptide Therapy Basics

A peptide is a polymer of amino acids joined by amide linkages called peptide bonds. Each peptide bond forms when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of the next, releasing a single water molecule. The result is a chain with a free amino terminus (N-terminus) on one end and a free carboxyl terminus (C-terminus) on the other, with amino-acid side chains projecting outward and dictating folding, charge, hydrophobicity and receptor binding.

The line between a peptide and a protein is conventional rather than absolute. Most authors treat chains of fewer than about 50 amino acids as peptides and longer chains as proteins, though some textbooks draw the line at 40 or even 100 residues. The relevant difference is functional: small peptides typically adopt limited secondary structure (short alpha-helices, beta-turns) and act primarily as signaling molecules, while proteins fold into stable tertiary structures and often act as enzymes, antibodies or structural scaffolds [1][3].

Naming conventions you will see

• BPC — Body Protection Compound (e.g., BPC-157). Fragments isolated from human gastric juice.
• CJC — ConjuChem peptides (e.g., CJC-1295). Modified GHRH analogs with extended half-lives.
• GHRP — Growth-Hormone Releasing Peptide (e.g., GHRP-6, ipamorelin). Ghrelin-mimetic secretagogues.
• TB — Thymosin Beta (e.g., TB-500). Actin-binding repair peptides.
• Numbered fragments — Numbers refer to the position of the parent sequence (e.g., PE-22-28 is amino acids 22 through 28 of a parent neuropeptide).

This nomenclature is messy because peptide research grew out of dozens of independent labs over five decades. There is no central naming authority — when you read a peptide name, the prefix usually identifies the discovering laboratory or the parent peptide family, and the number identifies the specific sequence or fragment.

Chemically, peptides and proteins are built from the same 20 proteinogenic amino acids and the same peptide bond. The distinction is one of size, folding behavior, manufacturing and pharmacology.

Size and structure

A short peptide — say oxytocin, which is only nine amino acids — has no real tertiary structure. It is a flexible string that adopts a binding-competent conformation when it docks into its receptor. A protein like albumin (585 amino acids) folds into a defined three-dimensional architecture stabilized by disulfide bonds, hydrogen bonds and hydrophobic packing.

Manufacturing

Peptides under roughly 50 amino acids can be made by solid-phase peptide synthesis (SPPS), the chemistry pioneered by Bruce Merrifield in 1963. Each amino acid is added stepwise to a growing chain anchored to a resin bead. This is fully chemical, scalable and produces a sequence-defined product. Proteins of significant size must be produced by recombinant DNA expression in bacteria, yeast or mammalian cell culture, which introduces the risk of host-cell impurities and requires extensive downstream purification [2][3].

Pharmacology and stability

Peptides usually act on a single cell-surface G-protein-coupled receptor (GPCR) or ion channel, producing a specific signaling response. Proteins like monoclonal antibodies often work by binding and neutralizing or by recruiting immune effectors. Peptides are typically cleared faster (minutes-to-hours half-life in unmodified form) because plasma peptidases such as DPP-4 and neutral endopeptidase rapidly cleave them. Modern peptide drugs solve this by adding a fatty-acid tail for albumin binding (semaglutide, tirzepatide, retatrutide) or by PEGylation, extending the half-life from hours to days [3][4].

Most therapeutic peptides act as agonists or antagonists at cell-surface receptors. They mimic or block an endogenous signal — a hormone, neuropeptide or growth factor — and engage downstream second-messenger cascades (cyclic AMP, calcium, MAP kinases) that change cell behavior.

A useful example is semaglutide, a long-acting analog of the gut hormone GLP-1. GLP-1 is released from intestinal L-cells after a meal and binds the GLP-1 receptor on pancreatic beta-cells, stomach smooth muscle and hypothalamic neurons. Receptor activation increases insulin secretion in a glucose-dependent manner, slows gastric emptying and reduces appetite. Semaglutide reproduces all three effects but, because it carries two amino-acid substitutions (Aib8 and Arg34) plus a C18 fatty-acid chain, it resists DPP-4 cleavage and binds albumin tightly, extending its half-life to roughly seven days [5][6].

That high specificity is what gives peptide drugs their characteristically clean target profile. Unlike small molecules, which often hit several off-target proteins because of their compact size, a 30-residue peptide makes a large surface contact with its receptor and rarely binds anywhere else. The trade-off is that peptides cannot easily cross cell membranes — they bind only what is exposed on the cell surface or in the extracellular space [3].

Mechanism categories

• Hormonal mimetics — semaglutide (GLP-1), tesamorelin (GHRH), sermorelin (GHRH).
• Receptor agonists with novel signal bias — tirzepatide (dual GIP/GLP-1, biased toward cAMP over beta-arrestin recruitment at GLP-1R) [7].
• Local tissue-repair signals — BPC-157, TB-500 modulate angiogenesis and nitric-oxide signaling at sites of injury.
• Mitochondrially-encoded signaling peptides — MOTS-c regulates AMPK activity and metabolic flexibility [8].

The fastest-growing class of peptide therapeutics is the incretins — gut-derived peptides released in response to nutrient intake that potentiate insulin secretion and reduce food intake. Two physiologic incretins exist in humans: glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). A related peptide, glucagon, raises blood glucose and energy expenditure and is being deliberately re-engineered into multi-agonist drugs because its receptor activates thermogenesis and hepatic lipid oxidation [5][9].

The clinical incretin lineup

• GLP-1 receptor agonists — semaglutide (FDA-approved as Ozempic, Wegovy, Rybelsus) is the canonical drug in this class. It produces ~15% mean weight loss at 68 weeks in STEP trials and has cardiovascular-event benefit in SELECT.
• Dual GIP/GLP-1 agonists — tirzepatide (FDA-approved as Mounjaro, Zepbound) combines GIP and GLP-1 agonism in a single 39-amino-acid peptide and produces ~21% mean weight loss in SURMOUNT-1. The GIP receptor activity is at native potency, while GLP-1 receptor binding is intentionally weakened roughly five-fold — an imbalance designed to retain efficacy while improving GI tolerability [7].
• Triple GIP/GLP-1/glucagon agonists — retatrutide adds glucagon-receptor agonism for thermogenic effect and has produced ~24% weight loss at 48 weeks in TRIUMPH-2 trials.
• Investigational glucagon/GLP-1 dual agonists — survodutide, mazdutide are advancing through Phase 3.
• Amylin co-agonists — cagrilintide (paired with semaglutide as CagriSema) targets the amylin receptor for additive satiety.

The incretin pipeline has effectively redefined obesity medicine. As of 2026 it accounts for the majority of new pharmacologic weight-loss approvals and a growing portion of cardiovascular-outcomes research.

Outside the incretin space, peptide research clusters into several therapeutic categories. Each has its own canonical reference compounds.

Growth-hormone secretagogues

These peptides stimulate the pituitary to release endogenous growth hormone (GH) instead of injecting recombinant GH directly. They fall into two pharmacologic families: GHRH analogs (which act on the GHRH receptor — examples include sermorelin, CJC-1295, tesamorelin) and ghrelin-mimetic GHRPs (which act on the ghrelin/GHS receptor — examples include ipamorelin, GHRP-2, GHRP-6). The two families are often stacked, because they act through complementary mechanisms and produce a larger, more physiologic GH pulse than either alone. CJC-1295 plus ipamorelin is the canonical pairing.

Healing and tissue-repair peptides

BPC-157 is the most-studied compound in this class. It is a 15-amino-acid fragment of a protein originally isolated from human gastric juice and shows wide-ranging effects on angiogenesis, nitric oxide signaling, dopamine and serotonin pathways, and tendon-ligament healing in rodent models [10][11]. TB-500, a synthetic 17-amino-acid fragment of thymosin beta-4, upregulates actin polymerization and is studied for similar repair indications. ARA-290 targets the tissue-protective erythropoietin receptor complex.

Cognitive and neuro-active peptides

Selank and Semax are synthetic peptides derived from regulatory fragments of ACTH and tuftsin, developed in Russia and approved there for anxiety and stroke recovery respectively. They are not FDA-approved. DSIP (delta-sleep-inducing peptide) and cerebrolysin (a mixture of low-molecular-weight neuropeptides) sit in the same category.

Mitochondrial peptides

A newer class encoded within the mitochondrial genome itself. MOTS-c is the most-studied member and was characterized by Lee and colleagues as an exercise-induced regulator of metabolic homeostasis [8]. SS-31 (elamipretide) targets the inner mitochondrial membrane.

Bioregulator peptides

Short di-, tri- and tetrapeptides — epitalon, vesugen, livagen — that originated in the Khavinson laboratory in Russia and are studied for tissue-specific gene-expression effects.

Peptides cannot generally be taken as an oral pill in the conventional sense. The acidic gastric environment, brush-border peptidases and intestinal first-pass metabolism degrade most peptides before they can be absorbed. Oral bioavailability of an unmodified peptide is typically <1%. Pharmacology therefore depends on choosing the right route [3][12].

Subcutaneous (SC) injection

The dominant route for therapeutic peptides. The peptide is injected into the loose connective tissue under the skin (abdomen, thigh, upper arm) using a 4–8 mm insulin needle. Bioavailability is typically 70–100%, and absorption is reproducible because the SC depot is well-vascularized but lacks the gut's enzymatic load. Most weekly incretins and daily GH peptides are given this way [12].

Intramuscular (IM) injection

Less commonly used for peptides because absorption is faster but no more bioavailable than SC. Reserved for specific compounds — e.g., depot formulations or volumes too large for SC.

Intranasal

Used for small, lipophilic peptides such as PT-141 (bremelanotide), Selank, Semax and oxytocin. Bioavailability is variable (5–30%) and depends heavily on formulation. Intranasal administration provides partial bypass of the blood-brain barrier via the olfactory and trigeminal pathways, which is why several neuropeptides are formulated this way.

Oral and sublingual

Only a handful of peptides have been engineered for oral delivery — Rybelsus (oral semaglutide) uses the absorption enhancer SNAC to push bioavailability up to ~1%, which is sufficient given semaglutide's potency but inefficient. Most peptides marketed as oral or sublingual capsules in the consumer space have minimal systemic bioavailability and are unlikely to reproduce injectable pharmacology.

Topical and intra-articular

Used for cosmetic peptides (e.g., GHK-Cu, SNAP-8) and for site-of-injury delivery of healing peptides in some experimental protocols. Systemic exposure is minimal.

It is important to be precise about regulatory status. A small set of peptides have full FDA approval and are prescribed by clinicians — semaglutide, tirzepatide, tesamorelin, bremelanotide, oxytocin, leuprolide, octreotide, exenatide, liraglutide and several dozen others [2]. A second set — sermorelin, hCG — were once approved but withdrawn from the marketed-drug list and are now obtainable only through compounding pharmacies under the FDA 503A/503B framework.

The much larger category — including BPC-157, CJC-1295, ipamorelin, TB-500, MOTS-c, retatrutide, the bioregulators and most cognitive peptides — has no FDA approval and no marketing authorization in the EU, UK, Canada or Japan. They are sold in the United States only as research chemicals for laboratory use, with explicit not-for-human-consumption labeling. FDA placed BPC-157 and several other peptides on its 503A bulks-list rejection notice in 2023, formally barring compounding pharmacies from preparing them for human prescription dispensing.

Researchers and consumers in this space should be aware of three points. First, research-grade material is not held to the analytical standards of pharmaceutical product — purity, sequence verification, endotoxin levels and excipient identity can vary across suppliers. Second, dosing protocols circulated online are extrapolated from animal studies, small open-label series and anecdotal use rather than from registered clinical trials. Third, importing or possessing some of these compounds for personal use may have legal consequences depending on jurisdiction. This guide is written for informational and research purposes only; it is not medical advice and does not constitute a recommendation to self-administer any of the compounds discussed.

Most peptide protocols follow a common structure. Understanding the vocabulary helps you read them critically.

• Compound and salt form — e.g., "BPC-157 acetate." The salt affects solubility and is rarely clinically relevant at the doses used.
• Vial size — milligrams of lyophilized peptide in the vial. Common sizes are 2 mg, 5 mg, 10 mg.
• Reconstitution volume — milliliters of bacteriostatic water added to dissolve the powder. See our reconstitution guide for the math.
• Concentration — milligrams per milliliter. Derived as vial mg ÷ reconstitution mL.
• Dose — typically expressed in micrograms (mcg) or milligrams (mg) per administration.
• Frequency and duration — once daily, twice daily, weekly. Cycles often run 4–12 weeks followed by a washout.
• Route — subcutaneous (SC) is dominant; intramuscular (IM) and intranasal also appear.
• Storage — lyophilized vials are typically refrigerated; reconstituted solutions go to 2–8 °C and are used within 28 days. See our storage guide.

Translating a dose in micrograms into a number of insulin-syringe units requires the concentration: Units = (Dose in mg ÷ Concentration in mg/mL) × 100. Our syringe measurement guide walks through this conversion with worked examples.

What good supplier documentation looks like

A reputable research-peptide supplier should provide a certificate of analysis (COA) with every lot. The COA confirms identity by mass spectrometry (the measured molecular weight should match the theoretical mass to within a few daltons), purity by HPLC (typically reported as area-under-curve at 214 nm, with research-grade lots above 95% and pharmaceutical grade above 99%), water content by Karl Fischer titration, and absence of bacterial endotoxin by LAL assay. If the supplier cannot produce a current COA for the specific lot number on your vial, that is a meaningful red flag. The COA is also where you confirm the salt form (acetate, TFA, hydrochloride) — this matters because the mass on the label is the gross mass including counterion, and a TFA salt can carry 10–15% of the labeled mass as trifluoroacetate rather than peptide.

How peptide research evolved

Modern peptide pharmacology rests on three foundational discoveries. Insulin, isolated by Banting and Best in 1921, was the first peptide hormone purified and used therapeutically. Solid-phase peptide synthesis, invented by Bruce Merrifield in 1963 (Nobel Prize 1984), made arbitrary sequence-defined peptides chemically accessible. Recombinant DNA technology, developed through the 1970s, allowed production of larger peptides and proteins in microbial culture. Together they enabled a six-decade expansion of peptide therapeutics from a handful of natural products to the more than 80 currently approved drugs and several hundred candidates in active clinical development [1][2][3].