Pinc PeptideThe girly peptide research blog

Guide · updated 10 October 2026

What are peptides? The science in plain English

Peptides are short chains of amino acids, the same building blocks that make up proteins12. The body makes many of its own, such as the gut hormone GLP-1, which helps control appetite and blood sugar, and LL-37, which kills bacteria14151617. Most peptides covered here are lab-made copies, pieces or reworked versions of natural ones181920.

124 sources · 17 min read · By the Pinc Peptide editors

Watch in 1 minute 42 seconds. How amino acids join into peptides, where a peptide ends and a protein begins, which peptides the body makes, and why most peptide drugs are injected. No sound: every word is on screen, and the video as text has them all.
The video as text

  1. 0:00

    A round badge opens over a drifting field of molecules: “What are peptides? The science in plain English”. Lettered beads circle it, spelling PEPTIDE in one-letter amino-acid codes.

  2. 0:08

    “20 kinds of amino acid”: twenty lettered beads drop into a row in five coloured groups, hydrophobic, polar, acidic, basic, and glycine / proline. They gather into a ring between two badges, “Some made by the body” and “The rest from food”.

    Proteins use 20 different amino acids. The body makes some of them. The rest, called essential amino acids, must come from food.

    Sources on screen: NHGRI Talking Glossary: “Amino Acids”

  3. 0:19

    Six beads, G, A, S, E, K and L, link into a chain, and a tag names one link “peptide bond”. A close-up shows the acid group (CO and OH) of one amino acid meeting the amino group (N and H) of the next: OH and H leave as water, H₂O, and a peptide bond joins the two.

    Amino acids join in a chain. Each link is called a peptide bond. Each bond joins the acid group of one amino acid to the amino group of the next, giving off a molecule of water.

    Sources on screen: OpenStax, Biology 2e, section 3.4: “Proteins” (2018)

  4. 0:29

    Fifteen lettered beads, G E P P P G K P A D D A G L V, drop in one by one and link into a chain that bends into a wave. A tag reads “BPC-157 · 15 amino acids”, and the beads are numbered 1 to 15.

    The order of the amino acids, called the sequence, sets a chain’s shape and what it does. BPC-157, for example, is a chain of 15.

    Sources on screen: OpenStax, Biology 2e, section 3.4 (2018) · FDA briefing document on BPC-157 (2026)

  5. 0:39

    Seven compounds drawn as rows of beads, one bead per amino acid: GHK-Cu 3, ipamorelin 5, TB-500 7, BPC-157 15, semaglutide 31, tirzepatide 39 and tesamorelin 44. A white card behind them reads “peptide: typically 2 to 50”. A dashed line at 40 reads “US rule: over 40 is a protein”, and tesamorelin, past the line, is tagged “protein”.

    A peptide is typically 2 to 50 amino acids long. Since 2020, US rules count any chain of more than 40 as a protein.

    Sources on screen: NHGRI Talking Glossary: “Peptide” · Federal Register, 85 FR 10057 (2020) · each compound’s own sources

  6. 0:50

    Insulin’s 110 beads run round a loop, tagged “signal · 24”, “B chain · 30”, “C peptide” and “A chain · 21”, with its cysteines (C) in amber. The signal and the C peptide drop away, the B and A chains line up in two rows joined by three sulfur bridges, and a tag adds them up: “30 + 21 = 51”, protein.

    Insulin starts as one chain of 110 amino acids. It is then cut into two chains, of 30 and 21, joined by sulfur bridges. US rules add them up: 51, so insulin counts as a protein.

    Sources on screen: UniProt P01308, human insulin · Federal Register, 85 FR 10057 (2020)

  7. 1:01

    A woman in a rose sweater, with four badges, each linked by a line to a spot on her body. GHRH (hormone): A brain hormone, 44 amino acids long, that tells the pituitary gland to release growth hormone. Glutathione (antioxidant): Just three amino acids long, it is the most abundant antioxidant the body makes. GLP-1 (hormone): A gut hormone that helps control blood sugar and appetite. LL-37 (defender): Released into sweat and wound fluid, it kills bacteria by punching holes in their membranes.

    A number of hormones and other chemical messengers are peptides, far smaller than proteins. Others protect the body.

    Sources on screen: Nobel Prize press release (1984) · Science (1982) · FDA briefing on CJC-1295 (2024) · Eur J Nutr (2015) · Cancer (2014) · Wegovy prescribing information · EMA Mounjaro overview (2026) · UniProt P49913 · J Clin Invest (2003)

  8. 1:12

    A scientist with round glasses holds up a test tube. Kisspeptin-10, “10 amino acids, unmodified”, Y N W N S F G L R F, beside a stopwatch: half-life in blood about 4 min. Semaglutide, “31 amino acids + a fatty chain”, its fatty chain gripping albumin, beside a calendar: half-life in blood about 1 week.

    Unmodified peptides often last only minutes in the blood. Semaglutide’s added fatty chain grips albumin, a blood protein, so its half-life is about a week.

    Sources on screen: FDA briefing document on kisspeptin-10 (2024) · J Med Chem (2015) · Wegovy prescribing information

  9. 1:24

    A short chain of beads in a stomach; enzymes bite through its links. Then two panels of 100 dots, each dot 1% of the dose. Tablet: 1–2% of a semaglutide tablet’s dose reaches the blood. Injection: 89% of a dose injected under the skin.

    Gut enzymes break peptides down. Only about 1% to 2% of a semaglutide tablet’s dose reaches the blood, against 89% of an injection.

    Sources on screen: Nature Reviews Drug Discovery (2020) · Wegovy and Rybelsus prescribing information

  10. 1:34

    The Pinc Peptide mark, one amino acid in ball and stick, builds itself atom by atom above the wordmark. Then: “Read the full guide: What are peptides? The science in plain English. Educational only. Not medical advice.”

What are peptides made of?

Peptides are built from amino acids. Proteins use 20 different amino acids; the body can make some of them, but the rest, called essential amino acids, must come from food2.

In a peptide, the amino acids are joined in a row by peptide bonds. Each bond forms when the acid (carboxyl) group of one amino acid links to the amino group of the next, giving off a molecule of water3.

The order of the amino acids, called the sequence, sets a chain’s shape and what it does, much as the order of letters sets the meaning of a word3. Scientists write a sequence with one letter per amino acid: BPC-157, for example, is GEPPPGKPADDAGLV, a chain of 1521.

Many peptides also carry small extras192223:

  • Caps on the ends: TB-500 has an acetyl group, a small chemical cap, on its first amino acid, and sermorelin ends in an amide group192425.
  • Rings: melanotan II is a chain of seven amino acids closed into a ring22. Bremelanotide (PT-141), sold as the approved drug Vyleesi, has the same ring and differs only at one end of the chain222627.
  • Unusual building blocks: semaglutide has Aib (aminoisobutyric acid) in place of one amino acid, which helps protect it from an enzyme that breaks down GLP-11520. Three of ipamorelin’s five amino acids are not used in human proteins as such, including D-phenylalanine, the mirror-image form of phenylalanine2328.

Peptide or protein: where is the line?

Mostly, the difference is size. A peptide is typically 2 to 50 amino acids long, and a longer chain, of 51 or more, is called a polypeptide1. A protein is made of one or more long chains folded into a particular shape4.

The cutoff is a convention, so rules draw it in different places. US federal rules in force since 23 March 2020 count any chain of more than 40 amino acids as a protein, which puts a medicine of that size under the separate rules for biological products29.

Insulin shows how blurry the line is. The body first makes it as one chain of 110 amino acids, then cuts out pieces to leave two chains, of 30 and 21, held together by two sulfur bridges5. A 2020 review of peptide medicines counts insulin among them6, but the US rule adds the two chains together, 51 amino acids in all, so insulin is a protein529. On 23 March 2020 the approvals of insulin brands such as Humalog, Lantus and NovoLog became biologics licenses30.

Among the compounds on this site, only tesamorelin, a chain of 44, and thymosin beta-4, a chain of 43, are over the US line; tirzepatide and retatrutide, at 39 each, sit just under it293132333435. In a 2026 warning letter, FDA told a company that its thymosin beta-4 was a biological product, which needs a biologics license before it can be sold36.

What do peptides do in the body?

A number of hormones and other signaling molecules are peptides, far smaller than proteins7. Hormones are released into the blood and tell other organs what to do1437:

  • Insulin lowers blood sugar5. Amylin, which the pancreas releases along with insulin when you eat, helps you feel full3839.
  • GLP-1, a gut hormone, helps control blood sugar and appetite141516.
  • GHRH (growth-hormone-releasing hormone), a chain of 44 amino acids, is the brain hormone that tells the pituitary gland to release growth hormone4041.
  • Kisspeptin switches on the reproductive system: people born with a broken receptor for it do not go through puberty normally374243.
  • Alpha-MSH (alpha-melanocyte-stimulating hormone) tells pigment cells to make melanin and damps down inflammation4445.

Others protect the body174647:

  • LL-37 is stored in white blood cells and released by skin and airway cells into sweat and wound fluid, where it kills bacteria by punching holes in their membranes174849.
  • Glutathione, a chain of just three amino acids, is the most abundant antioxidant the body makes and mops up reactive oxygen molecules4650.
  • Thymosin alpha-1, first isolated from calf thymus in 1977, is thought to activate immune cells1847.

Some natural peptides are cut from bigger proteins. LL-37 is the last 37 amino acids of a 170-amino-acid precursor, thymosin alpha-1 matches the front end of a larger protein called prothymosin alpha, and kisspeptin-10 is the last 10 amino acids of kisspeptin-54, the main form in blood17515253. MOTS-c, a chain of 16, is unusual: its code sits in the small, separate DNA of mitochondria, the cell’s power plants54.

Why are lab-made peptides changed?

Unmodified peptides often last only minutes in the blood. Blood enzymes break DSIP down within minutes, and half of a dose of kisspeptin-10, or of sermorelin, is gone in about 4 minutes42555657.

Drug designers rework peptides so they last longer2058:

  • Semaglutide is a version of GLP-1 with two amino acids swapped and a fatty-acid chain added20. The chain makes it stick to albumin, the main protein in blood, so its half-life, the time it takes for half of a dose to clear, is about a week1415.
  • Tirzepatide carries a similar fatty chain and has a half-life of about 5 to 6 days, so it is injected once a week59.
  • CJC-1295 with DAC adds one extra amino acid that locks onto albumin, and a single injection keeps working for about a week5860.

The peptides covered here fall into three rough groups171920:

  • Copies: LL-37, MOTS-c, thymosin alpha-1 (sold abroad as Zadaxin), thymosin beta-4, gonadorelin (the brain hormone GnRH) and glutathione are the same molecules the body makes1718325461626364.
  • Pieces: sermorelin is the first 29 of GHRH’s 44 amino acids, TB-500 is 7 of the 43 in thymosin beta-4, AOD-9604 is the tail end of human growth hormone plus one extra amino acid, and KPV is the last three of alpha-MSH19244565. MGF copies the 24-amino-acid tail of one form of the growth factor IGF-1, and PEG-MGF is sold as MGF with a chain of polyethylene glycol (PEG) attached666768. BPC-157 is described as a piece of a protein from human gastric juice, and pentadeca arginate is BPC-157 sold as an arginine salt21697071.
  • Reworked versions: semaglutide is a reworked GLP-1, CJC-1295 is GHRH’s first 29 amino acids with four swapped, melanotan II is a ring-shaped version of alpha-MSH, and selank and semax add a three-amino-acid tail to short natural sequences so they last longer20417273. Tirzepatide is a lab-made peptide that acts on the receptors for two gut hormones, GIP and GLP-11659.

Not every natural origin is well established: a 2006 review noted that no gene, precursor protein or receptor had been found for DSIP, the nine-amino-acid “delta sleep-inducing peptide”7475.

How are peptides made?

Cells build peptides and proteins from instructions in genes, which spell out the order of the amino acids2.

Chemists can build a peptide one amino acid at a time. In solid-phase synthesis, the method that won Bruce Merrifield the 1984 Nobel Prize in Chemistry, the first amino acid is anchored to a solid polymer, leftovers are washed away after each new amino acid is added, and the finished chain is cut free only at the end7. By 1984, thousands of different peptides had been made this way, and machines that run the process automatically were already on sale7.

Peptides sold online often come as freeze-dried powder in vials labeled for research use only767778. One seller’s page claims purity of at least 95%, “typically verified by HPLC and mass spectrometry”, two lab methods for separating and identifying molecules78. Independent tests find many failures: Finnrick, a testing company, failed 92 of its latest 200 BPC-157 samples and 63 of its latest 200 retatrutide samples, as of 30 September 20267980. Finnrick’s users choose what to send, so these results are not a random sample8182.

Small differences in form make different products. BPC-157 is also sold as an acetate salt, which FDA treats as a separate ingredient, and the name CJC-1295 covers two molecules: the bare 29-amino-acid chain and a version with an extra albumin-binding amino acid21415860.

Why are most peptides injected?

Peptide and protein medicines usually have to be injected, because the digestive tract is a strong barrier: its enzymes break peptides down, and its lining lets little through6.

Pills are possible but inefficient. Semaglutide tablets contain SNAC, an ingredient that helps the stomach absorb the drug, yet only about 1% to 2% of a tablet’s dose reaches the blood, compared with 89% of an injection under the skin1483. A 25 mg Wegovy tablet taken every day is predicted to give blood levels similar to a 2.4 mg injection once a week14.

Oral versions of peptides sold online are often untested: FDA found no human data on BPC-157 or ipamorelin taken by mouth, though both are marketed as capsules or tablets2128.

MK-677, sold for the same growth-hormone goals as several of these peptides, is taken as a pill; it is a small drug molecule, not a peptide84.

What are collagen peptides?

Collagen peptides, sold as supplements, are hydrolyzed collagen: a mix of small peptides made by breaking collagen down with enzymes812. The collagen comes from cattle and pigs, or from the skin, scales and bones of fish8.

Digestion breaks most peptides down6, yet some collagen fragments reach the blood. After volunteers swallowed 9.4 to 23 g of gelatin hydrolysate, pieces two or three amino acids long, mainly one called Pro-Hyp (proline joined to hydroxyproline), rose in their blood and peaked 1 to 2 hours later9.

Whether they help skin is disputed. Pooled analyses of 19 and 26 trials found better skin hydration and elasticity than with a placebo; in the first, 95% of the 1,125 people were women1011. A 2025 analysis of 23 trials with 1,474 people also found gains in hydration, elasticity and wrinkles overall, but none in the trials without drug-company funding or in the higher-quality trials12. Its authors concluded that no clinical evidence supports using collagen supplements to prevent or treat skin aging12.

In the US, supplement makers are responsible for checking their products’ safety and labeling before sale, and FDA can act against a product after it reaches the market13. Our collagen peptides guide covers the trials for skin, joints, bones and muscle, and who paid for them.

The peptides on this site, by size

Counted in amino acids, all but two of the single peptides covered here fall under the US 40-amino-acid line for proteins293133:

  • 3 amino acids: GHK-Cu (with a copper ion attached), KPV and glutathione, whose first link is an unusual side-chain bond rather than a standard peptide bond45468586.
  • 4 amino acids: epitalon and SS-31 (elamipretide)8788.
  • 5 amino acids: ipamorelin23.
  • 6 amino acids: GHRP-2, GHRP-6 and hexarelin899091.
  • 7 amino acids: TB-500, selank, semax, melanotan II and PT-141; the last two are closed into rings1922269293.
  • 9 amino acids: DSIP and oxytocin, whose first six amino acids a sulfur bridge closes into a ring749495.
  • 10 amino acids: gonadorelin, and kisspeptin-10, a piece of the 54-amino-acid kisspeptin-5453626396.
  • 13 amino acids: melanotan I9798.
  • 15 amino acids: BPC-157, also sold as an arginine salt called pentadeca arginate217071.
  • 16 amino acids: AOD-9604 and MOTS-c546599.
  • 24 amino acids: MGF, which sellers offer with a PEG chain attached as PEG-MGF; we found no public record of PEG-MGF’s full structure68100.
  • 28 amino acids: thymosin alpha-1 and VIP1851101102.
  • 29 amino acids: sermorelin, CJC-1295 and survodutide; CJC-1295 with DAC adds a 30th244158103104.
  • 31 amino acids: semaglutide and liraglutide1415105.
  • 34 amino acids: mazdutide106107.
  • 37 amino acids: LL-37 and cagrilintide17108.
  • 39 amino acids: tirzepatide and retatrutide3435.
  • 43 amino acids: thymosin beta-4, over the line3233109.
  • 44 amino acids: tesamorelin, also over the line2931.
  • Mixtures, not single peptides: cerebrolysin, digested pig brain protein that leaves small peptides and free amino acids, and thymalin, a calf thymus extract its developers call a complex of peptides110111112.
  • Peptide-like: dihexa, a small lab-made molecule that keeps two amino acids of the hormone fragment angiotensin IV and caps both ends113114.
  • Not peptides: NAD+ contains no amino acids, and MK-677 and orforglipron are small drug molecules84115116.

The evidence at a glance

TierBest human evidenceCompounds
ATwo or more large human trialsCagrilintide, Liraglutide, Mazdutide, Melanotan I (afamelanotide), Orforglipron, Oxytocin, PT-141 (bremelanotide), Retatrutide, Semaglutide, Survodutide, Tesamorelin, Thymosin alpha-1, Thymosin beta-4, Tirzepatide
BAt least one controlled human trialAOD-9604, BPC-157, Cerebrolysin, DSIP (emideltide), Epitalon, GHK-Cu, GHRP-2 (pralmorelin), GHRP-6, Glutathione, Gonadorelin, Hexarelin, Ipamorelin, Kisspeptin, LL-37, Melanotan II, MK-677 (ibutamoren), NAD+, Selank, Semax, Sermorelin, SS-31 (elamipretide), Thymalin, VIP (vasoactive intestinal peptide)
COnly small or uncontrolled human studiesCJC-1295
DAnimal studies onlyDihexa, KPV, MOTS-c, Pentadeca arginate (PDA), TB-500
ELab or anecdotal evidence onlyPEG-MGF

A tier grades the best human evidence for any use, not whether a compound is safe. Whether each one works for the use people take it for, and its legal status: the evidence table. How we grade: our method.

Questions people ask

What are peptides in simple terms?

Peptides are short chains of amino acids, typically 2 to 50 of them, linked in a row like beads on a string1. The body uses many as signals, such as the gut hormone GLP-1, which helps control appetite and blood sugar1416.

What is the difference between a peptide and a protein?

Mainly size: a peptide is typically 2 to 50 amino acids long, while a protein is made of one or more longer chains folded into a shape14. Under US federal rules, any chain of more than 40 amino acids counts as a protein29.

What are peptides used for?

Some are approved medicines: semaglutide, for example, is approved for type 2 diabetes and for weight management1483. Many others are sold by clinics and websites, mostly without approval, for weight loss, injury recovery, muscle, skin or slower aging, often with little or no evidence in people1928117118.

Is insulin a peptide?

It sits on the line. Each of its two chains, of 30 and 21 amino acids, is peptide-sized, and a 2020 review of peptide medicines includes it56. But US rules add the chains together, 51 in all, so insulin counts as a protein and is licensed as a biological product2930.

Are peptides natural?

Many are: the body makes insulin, GLP-1 and LL-37, and GHK, the peptide in GHK-Cu, occurs naturally in human blood5141785. Most peptides sold as drugs or online are lab-made, either as copies of natural peptides or as changed versions not found in nature182072.

Can you take peptides by mouth?

Some can be, but most are poorly absorbed, because the gut breaks peptides down and lets little through6. Even semaglutide tablets, which contain an absorption enhancer, get only about 1% to 2% of the dose into the blood14. Collagen peptides are eaten as supplements, and a few pieces of them two or three amino acids long reach the blood9. FDA found no human data on BPC-157 or ipamorelin taken by mouth2128.

What are peptides in skin care?

They are short amino acid chains added to skin creams; one popular example is GHK-Cu, a copper complex that appears on cosmetic labels as copper tripeptide-1117119. Small skin trials of GHK-Cu had mixed results, and we found no published study that injected it on its own in people, though many buyers now inject vials of it117120121122. Our guide to peptides in skincare covers the trials of Matrixyl, Argireline and copper peptides.

Are NAD+ and MK-677 peptides?

No. NAD+ is a coenzyme, a helper molecule that cells need to turn food into energy, and it contains no amino acids115123. MK-677, also called ibutamoren, is a small drug molecule taken as a pill84124.

Go deeper

Sources

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  3. 3Biology 2e, section 3.4: Proteins OpenStax, Rice University, 28 Mar 2018. openstax.org
  4. 4Protein (Talking Glossary of Genomic and Genetic Terms) National Human Genome Research Institute (NIH), accessed 7 Oct 2026. genome.gov
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  6. 6Advances in oral peptide therapeutics Nature Reviews Drug Discovery, Apr 2020. pubmed.ncbi.nlm.nih.gov
  7. 7Press release: The Nobel Prize in Chemistry 1984 The Royal Swedish Academy of Sciences, via NobelPrize.org, 17 Oct 1984. nobelprize.org
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  9. 9Identification of food-derived collagen peptides in human blood after oral ingestion of gelatin hydrolysates Journal of Agricultural and Food Chemistry, 10 Aug 2005. pubmed.ncbi.nlm.nih.gov
  10. 10Effects of hydrolyzed collagen supplementation on skin aging: a systematic review and meta-analysis International Journal of Dermatology, Dec 2021. pubmed.ncbi.nlm.nih.gov
  11. 11Effects of Oral Collagen for Skin Anti-Aging: A Systematic Review and Meta-Analysis Nutrients, 26 Apr 2023. pubmed.ncbi.nlm.nih.gov
  12. 12Effects of Collagen Supplements on Skin Aging: A Systematic Review and Meta-Analysis of Randomized Controlled Trials The American Journal of Medicine, Sep 2025. pubmed.ncbi.nlm.nih.gov
  13. 13Dietary Supplements U.S. Food and Drug Administration, 1 Oct 2024. fda.gov
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  29. 29Definition of the Term “Biological Product” (final rule, 85 FR 10057) Federal Register, 21 Feb 2020. federalregister.gov
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  48. 48An angiogenic role for the human peptide antibiotic LL-37/hCAP-18 The Journal of Clinical Investigation, Jun 2003. pubmed.ncbi.nlm.nih.gov
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