Peptides 101

A peptide is a short chain of amino acids joined by peptide bonds — longer than a single amino acid, shorter than a protein, with the boundary conventionally drawn around fifty residues.^[1] Chain length is what separates a peptide from the small-molecule drugs most people are familiar with (aspirin, ibuprofen): small molecules are compact enough to be absorbed through the gut and often to cross into cells directly, while a peptide’s size and its vulnerability to digestive enzymes are exactly why most are given by injection rather than swallowed.^[1]

Length also varies enormously within “peptide.” BPC-157 is fifteen amino acids. Semaglutide is a thirty-one-residue modified analogue of the human hormone GLP-1, with a fatty-acid side chain attached specifically to extend how long it stays in circulation. Sequence, not just length, determines what a peptide does — two peptides of similar size can have almost nothing else in common.

For the fuller treatment of what “research peptide” means as a commercial label rather than a chemical one, see What Are Research Peptides? in the FAQ.

Most peptides act from outside the cell: they bind a receptor on the cell surface — commonly a G-protein-coupled receptor — and that binding triggers a signaling cascade inside the cell, rather than the peptide itself entering and acting directly the way many small-molecule drugs do.^[1] Retatrutide, for example, is built to activate three separate metabolic receptors at once (GLP-1, GIP, and glucagon), while ipamorelin is selective for a single one — the ghrelin/growth-hormone-secretagogue receptor — which is why it raises growth hormone without the appetite and cortisol effects broader secretagogues can produce.

Route and half-life both follow from that surface-receptor mechanism plus a peptide’s vulnerability to enzymatic breakdown. A swallowed peptide is mostly broken down in the stomach and gut, and little of what survives crosses into the blood, which is why injection dominates. Half-life — how long a peptide persists before being cleared — ranges from single-digit minutes to several days depending on structural modifications such as fatty-acid conjugation or amino-acid substitutions that resist enzymatic cleavage.^[1] That range is why one compound is dosed multiple times a day and another once a week.

The receptor-level mechanics behind this, worked through in more depth, are in Peptide Research Basics.

Prof. Peptide grades the evidence behind every compound on one four-level scale, from strongest to weakest:

1. Human RCT — a randomized, controlled trial in people, with a comparison group and a pre-registered protocol. This is the only tier that can support a causal claim about effect and dose in humans. Retatrutide’s Phase 2 obesity trial sits here.^[2]
2. Human Observational — human data without randomization or a placebo arm: open-label studies, case series, or older small cohorts. It can show what happened in people, but not what would have happened without the compound. Sermorelin’s class has this kind of data — a small, uncontrolled long-term study of a closely related GHRH analogue in older adults is a representative example.^[3]
3. Animal Model — in vivo work in a non-human species. It can establish a mechanism and a dose-response relationship in that species, but animal results frequently fail to translate to humans, and no amount of animal data substitutes for a human trial. BPC-157’s angiogenic mechanism work is predominantly at this tier.^[4]
4. In Vitro — cultured cells or a biochemical assay, no whole organism involved. It is the earliest, most mechanistic kind of evidence, useful for showing a plausible pathway exists and nothing more. GHK-Cu’s gene-modulation studies in cultured skin cells sit here.^[5]

Most compounds this site profiles sit at the Animal Model or In Vitro tier, and that is worth stating plainly rather than obscuring: reaching a completed human RCT requires a funded, regulator-supervised drug-development program, which is expensive and rare outside compounds a pharmaceutical company has taken through that process. The incretins — semaglutide, tirzepatide, retatrutide — are the exception precisely because they went through full commercial development. Most research peptides never entered that pipeline at all, which is a statement about funding and regulatory history, not necessarily about a compound’s underlying biology.

“Research use only” is a label describing how a product is sold, not a chemical property. It marks material sold for laboratory use — not evaluated, not approved, and not manufactured to pharmaceutical standards by default. The same molecule can exist on both sides of that line at once: semaglutide is an FDA-approved medicine sold under brand names, and it is also sold as research-grade powder. The chemistry does not change across that line; the regulatory status, the quality assurance, and the legal position all do.

All content on Prof. Peptide is for educational and research purposes only. The compounds profiled here are described as research-use-only materials; nothing on this site is medical advice, diagnosis, or a treatment recommendation, and compounds are framed in the context of the published research rather than as products for human use. See the full disclaimer for the details.

Subcutaneous injection — under the skin, into the fat layer — is the default route for most research peptides. It is technically simple, has a large absorptive surface area, and avoids the digestive breakdown that limits swallowing. BPC-157’s few published human reports used other injections instead: into the knee, into a vein, and around the bladder wall.

Intramuscular injection goes deeper, into muscle tissue — testosterone esters in a TRT context are a common example. For peptide hormones it is not reliably better absorbed: two hCG studies disagree, and a recombinant FSH study found no difference from subcutaneous.

Nasal delivery bypasses first-pass liver metabolism and can act quickly, though approved nasal peptides report low absorption, 2.8–4.1% by label. PT-141 was first studied intranasally, but the only product regulators have approved is the injectable Vyleesi.

Oral delivery is rare for peptides because stomach acid and gut enzymes break most of a dose down, and little of the rest is absorbed.^[1] The exceptions are engineered: three semaglutide tablets (Rybelsus, Ozempic tablets and Wegovy tablets) use an absorption enhancer and still absorb only 0.4–2%, while linaclotide acts on a gut receptor without needing to be absorbed. It is something a formulation team engineers around, not something peptides do by default.

The label figures behind these comparisons are set out in peptide pills vs nasal sprays vs injections, with the compound-by-compound record in can peptides be taken orally, which peptides come in pill form and which peptides come as a nasal spray.

A vial of research peptide typically ships as a lyophilized (freeze-dried) powder holding a fixed mass of compound. Adding bacteriostatic water dissolves it into a solution, and the volume of water added sets the concentration — more water means a lower concentration and more syringe units per dose, less water means the opposite, but the amount of peptide in any given dose is identical either way. The arithmetic behind that, and a worked example, is in the FAQ’s bacteriostatic-water explainer.

For the exact water volume and syringe units for a specific vial size and target dose, use the dosage calculator rather than doing the math by hand, and see the Insulin Syringes & Injection guide for needle selection, injection technique, and site rotation.

Unreconstituted, lyophilized powder is the stable form: refrigerated or frozen and protected from light, most research peptides hold for months to years. Some are also light-sensitive in a way that matters even before reconstitution — Melanotan II is a commonly cited example, conventionally kept in its original amber vial or wrapped in foil.

Once reconstituted, the peptide is in solution and far less stable: it is conventionally refrigerated (not frozen, which can degrade some peptides) and used within a bounded window. A widely used convention — mirroring general multi-dose-vial guidance for antimicrobial-preserved solutions — treats a bacteriostatic-water reconstitution as good for about 28 days refrigerated. That is a convention researchers commonly follow, not a figure any peptide-specific trial established; treat it accordingly.

A Certificate of Analysis is a lab’s report on one batch, and the panels on it answer different questions:

• HPLC purity — what fraction of the vial, by chromatographic peak area, is the intended compound versus contaminants or degradation products.
• LC-MS identity — mass-spectrometry confirmation that the molecule present actually matches the labeled compound. This is a different question from purity, and one does not substitute for the other.
• Endotoxin — a screen (typically a LAL assay) for bacterial toxins that survive even after the bacteria themselves are gone.
• Sterility — a screen for live microbial growth. Methodology varies here more than the other panels: a rapid preliminary screen and full compendial sterility testing are not the same claim, and a certificate should say which one it ran.
• Heavy metals — elemental screening, usually by ICP-MS, for lead, arsenic, cadmium, and mercury against a stated limit.

Prof. Peptide profiles the independent laboratories behind these reports — what each one claims, what its accreditation search actually found, and how to verify a certificate yourself — on the labs pages, including ILS Laboratories and Bioviridian. The Vendor Testing Index lays out which vendor uses which lab, method, and panel side by side.

This is the part that makes Prof. Peptide different from a vendor directory, so it is worth stating plainly. A certificate pass means a person actually opened the lab report and recorded only what it prints — the lab, its accreditation only where one is actually printed, the panels run, and the mechanism to verify the report independently. Where a lab publishes its own copy of a certificate, we check it byte-for-byte or field-by-field against what the vendor displays, rather than taking the vendor’s copy on faith. Where a lab claims an accreditation, we check the accrediting body’s own directory (A2LA, ANAB, PJLA) rather than the lab’s word for it.

Every testing claim on a vendor page falls into one of three buckets: verified against a certificate we state it and name the lab; asserted by the vendor but unverified we attribute it to them explicitly; false or contradicted we remove it. We do not assign vendors a numeric score or rank — the one curated list, our Featured Vendors page, is a dated editorial selection, not a computed ranking, and the same is true of our discount codes. The full standard is set out in the methodology page.

Peptides on this site cluster into eleven research areas:

• Metabolic & GLP-1 — Weight regulation and glucose control — the incretin family, where the human evidence is deepest.
• Recovery & Tissue Repair — Tendon, ligament, muscle, and gut healing — largely animal literature, often extensive.
• Performance & Energy — Growth-factor and cellular-energy pathways studied for physical performance.
• Growth Hormone — Secretagogues that raise endogenous GH rather than supplying it, with older clinical work behind several.
• Cognitive & Nootropic — Mood and cognition — a cluster where much of the human data comes from outside Western regulatory systems.
• Skin Health & Anti-Aging — Pigmentation and dermal signaling — mechanism well characterized, safety literature thin.
• Gut Health & Immunity — Barrier and immune-signaling peptides, including several with antimicrobial research interest.
• Sleep & Recovery — Sleep-architecture peptides — a small, mostly older literature.
• Longevity — Cellular-aging and antioxidant pathways, mostly preclinical.
• Bioregulators — Short Khavinson-class peptides with a distinct, largely Eastern-European research tradition.
• Sexual Health — Melanocortin and oxytocin-pathway compounds studied for sexual function.

From here:

• Browse the full Peptide Library — every profile follows the same evidence-hierarchy discipline described above.
• Run the Dosage Calculator for reconstitution and unit math on a specific vial.
• Search the FAQ for a specific question — dosing, side effects, legality, and more, organized by topic.
• Compare prices across vendors for a compound you’re researching.
• Check verified discount codes once you’ve picked a vendor.