Key takeaways
Peptides are short chains of amino acids joined by peptide bonds, usually 2 to about 50 units long. Proteins are longer chains that usually fold into stable shapes, but the cut-off is a convention, not a law of nature. In the body, many peptides act as signals, such as hormones that bind receptors on cells.
- 01Length is the main difference. Biology glossaries put peptides at 2 to 50 amino acids; FDA treats any chain over 40 as a protein.
- 02Many hormones are peptides. Insulin, GLP-1 and oxytocin all bind receptors on the surface of cells.
- 03Most peptide drugs are injected because digestion breaks them down. Oral semaglutide uses an absorption enhancer called SNAC.
- 04Peptides are made by chemistry or by cells. Solid-phase synthesis builds chains one amino acid at a time; recombinant methods use bacteria or yeast.
- 05Labels use mass or activity. Milligrams measure weight, units measure biological effect, and HPLC purity is not peptide content.
- 06Research use only peptides are not FDA-approved drugs. That label does not make them lawful to sell for human use.
This article belongs to our Peptide Basics & Science category, which covers the wider topic.
What is a peptide, chemically?
A peptide is a chain of amino acids held together by peptide bonds.
An amino acid is a small molecule with an amino group at one end, a carboxyl group at the other, and a side chain that gives it its character. Your cells build proteins from 20 standard amino acids.
When the carboxyl group of one amino acid reacts with the amino group of the next, they form an amide link, the peptide bond, and release one molecule of water. The IUPAC-IUBMB nomenclature rules for amino acids and peptides define a peptide as any compound made by this kind of amide formation. Each amino acid in the chain is called a residue. Sequences are written from the free amino end (N-terminus) to the free carboxyl end (C-terminus), so Gly-Ala and Ala-Gly are different molecules.
- Dipeptide: 2 residues.
- Tripeptide: 3 residues. GHK, the copper-binding peptide in many skin-care products, is one.
- Oligopeptide: a few residues, up to about 10 to 20 under the IUPAC-IUBMB rules.
- Polypeptide: a longer chain. Proteins are built from one or more.
Working out molecular weight
Each bond releases one water molecule (18.02 g/mol), and n is the number of amino acids. Example: glycine (75.07) + alanine (89.09) - 18.02 = 146.14 g/mol for Gly-Ala.
Free calculator
Peptide Molecular Weight Calculator. Paste a one-letter sequence to get average and monoisotopic mass, formula and composition.
Where does a peptide end and a protein begin?
There is no single cut-off. Different bodies draw the line in different places.
The NHGRI genetics glossary entry on peptides gives the most common rule of thumb: a peptide is typically 2 to 50 amino acids.
| Source | Where it draws the line | Why it matters |
|---|---|---|
| NHGRI glossary | Peptide: 2 to 50 amino acids. Polypeptide: 51 or more. | Teaching convention in biology |
| IUPAC-IUBMB | Oligopeptide: under about 10 to 20 residues. Polypeptide: more. | Naming convention for chemists |
| US FDA (21 CFR 600.3) | Protein: more than 40 amino acids. | Decides drug versus biologic regulation |
Under the FDA rule, chains that associate as they do in nature are counted together.
The line is fuzzy because length stands in for what scientists care about: shape. Longer chains usually fold into stable three-dimensional structures, but some short chains fold and many proteins are several chains held together.
Insulin has two chains of 21 and 30 amino acids, 51 in total, so FDA counts it as a protein. The FDA announcement on the insulin transition explains that insulin applications were deemed biologics licenses on March 23, 2020. Tirzepatide, a 39-amino-acid peptide, sits under the line and is regulated as a drug. See semaglutide vs tirzepatide.
How do peptides work in the body?
Most natural peptides are messengers that carry a signal from one cell to another.
Many peptides are hormones, chemical signals released into the blood. Others are neuropeptides, released by nerve cells. Together they help control blood sugar, appetite, pain, birth and breastfeeding.
Receptors work like a lock and key
Most peptides dissolve in water but not in the fatty membrane around a cell, so they cannot slip inside. Instead they bind a receptor, a protein in the cell membrane with one end facing out. When the peptide docks, the receptor changes shape and triggers events inside the cell.
Receptors are picky. A small change in sequence can make a peptide bind more tightly or last longer, which is how drug designers turn short-lived hormones into medicines.
| Peptide | Size | Molecular weight | Main job |
|---|---|---|---|
| Oxytocin | 9 amino acids | 1,007.19 g/mol | Uterine contractions in labor; milk let-down |
| GLP-1 (7-36) amide | 30 amino acids | 3,297.6 g/mol | Boosts insulin after meals, curbs glucagon, slows stomach emptying |
| Human insulin | 51 amino acids, two chains | 5,808 g/mol | Helps cells take up glucose |
Oxytocin and insulin weights are from the FDA-approved Pitocin and Humulin R labels. The GLP-1 value is the computed figure in PubChem.
Native GLP-1 lasts only about one to two minutes in the blood before an enzyme called DPP-4 (dipeptidyl peptidase-4) clips off its first two amino acids. That suits a hormone that responds to each meal. It is a problem for a medicine.
Why are most peptide drugs injected?
Your gut treats a peptide drug much like the protein in a meal: as food to break down.
Stomach acid and enzymes such as pepsin and trypsin cut peptide bonds. Peptides are also large and water-loving, so they cross the gut wall poorly. A 2022 review of therapeutic peptides in Signal Transduction and Targeted Therapy names poor membrane permeability and poor stability in the body as their two main drawbacks, and notes that most are injected as a result.
How chemists make peptides last longer
The goal is a longer half-life, the time it takes for the amount in the blood to fall by half. The Rybelsus prescribing information describes semaglutide as a GLP-1 analog with 94% sequence homology to the human hormone, built with two fixes:
- Resist the enzyme. Semaglutide is stabilized against DPP-4.
- Hitch a ride on albumin. A fatty acid side chain binds albumin, the main protein in blood. The label names this as the main reason the half-life is about one week.
The oral semaglutide exception
Rybelsus tablets contain salcaprozate sodium (SNAC), an absorption enhancer that the label says helps semaglutide be absorbed, mainly in the stomach. Even so, the label puts absolute bioavailability, the share that reaches the blood, at about 0.4% to 1% for the original tablets and 1% to 2% for the newer formulation. Absorption also depends on food and water, so the label gives strict empty-stomach instructions.
FDA also approved a once-daily semaglutide tablet under the Wegovy brand in December 2025. A few other peptides can be swallowed, such as desmopressin tablets and octreotide capsules, but they remain exceptions.
How are peptides made?
There are two main routes: build the chain with chemistry, or have living cells make it.
Solid-phase peptide synthesis
Solid-phase peptide synthesis (SPPS) was developed by Bruce Merrifield in the early 1960s and won him the 1984 Nobel Prize in Chemistry. The growing chain stays anchored to an insoluble resin bead, so leftover chemicals can be washed away after each step.
- Attach the C-terminal amino acid to a bead.
- Remove the protecting group on its amino end.
- Couple the next protected amino acid, wash, and repeat.
- Cut the finished chain free and purify it, usually by HPLC.
At 99% per step, 30 steps give 0.99^30 = 0.74, about 74%. At 90% per step, 100 steps leave about 0.003%, the Nobel press release example of why long syntheses were impractical.
Recombinant production
In recombinant production, the gene for a peptide is put into bacteria or yeast, which make it as they grow. Human insulin made in E. coli was approved in 1982 as Humulin, the first recombinant DNA drug approved by FDA. The Rybelsus label states that semaglutide's backbone is produced by yeast fermentation.
How are peptides measured and labeled?
Labels use two kinds of units, mass and biological activity, and mixing them up causes errors.
Mass: mg and mcg
Most peptides are labeled by mass. A milligram (mg) is one thousandth of a gram, and a microgram (mcg) is one thousandth of a milligram.
Example: 500 mcg = 0.5 mg, and 2.5 mg = 2,500 mcg.
Free calculator
mcg to mg Converter. Convert micrograms, milligrams and grams without slipping a decimal place.
Activity: IU and units
An international unit (IU) measures biological activity against a reference standard, not weight, and its size differs for every substance. Insulin is the best-known example, which is why syringes are marked in units; see how to read an insulin syringe. Converting units to milligrams needs a substance-specific factor, as in the HGH IU to mg calculator.
Molecular weight and charge
Molecular weight, in g/mol or daltons (Da), links mass to the number of molecules, which you need for molar concentrations. The 2022 review puts most therapeutic peptides between 500 and 5,000 Da.
The isoelectric point (pI) is the pH at which a peptide carries no net charge. Peptides tend to be least soluble near their pI, so chemists use it to choose buffers.
Free calculator
Isoelectric Point & Net Charge Calculator. Estimate a sequence's pI and net charge at any pH across four pKa scales.
Purity and content
HPLC (high-performance liquid chromatography) separates the target peptide from related impurities, such as chains missing one amino acid. A purity of 98% means the main peak is 98% of the peptide signal, usually read with UV light near 214 nm. Mass spectrometry confirms that the peak has the expected mass.
Purity is not the same as content
HPLC purity ignores water and counter-ions, salts such as acetate that pair with charged groups on the peptide. So 10 mg of freeze-dried powder can hold noticeably less than 10 mg of peptide. Net peptide content is measured separately.
How do approved peptide medicines differ from research peptides?
The same word covers FDA-approved medicines and unapproved chemicals sold online.
The 2022 review counts more than 80 peptide drugs approved worldwide. Familiar examples include:
- Insulins for diabetes.
- GLP-1 drugs such as liraglutide and semaglutide, and the dual GIP and GLP-1 agonist tirzepatide.
- Oxytocin (Pitocin) to start or strengthen labor contractions.
- Octreotide for acromegaly, leuprolide for advanced prostate cancer and teriparatide for osteoporosis.
Research peptides such as BPC-157 and TB-500 are chemicals sold online, often labeled for research use only. They are not FDA-approved drugs, and the label does not protect a seller who markets them to people. In an FDA warning letter to an online peptide seller in March 2026, the agency said website claims showed the products were meant for human use, making them unapproved new drugs.
In July 2026, an FDA advisory committee voted 8 to 6 to recommend adding BPC-157 to the list of bulk substances pharmacies may compound. The vote is not binding; FDA would have to complete rulemaking first. See BPC-157 research and legal status.
Unapproved means unverified
FDA has not reviewed an unapproved peptide for safety, effectiveness, identity, purity or sterility, and the vial may not match its label. This article is educational, not medical advice. Talk with a licensed clinician about any medicine.
Frequently Asked Questions
Are peptides and proteins the same thing?
Not quite. Both are chains of amino acids joined by peptide bonds. Peptides are the short ones, usually up to about 50 amino acids, and proteins are longer chains that usually fold into a stable shape. The boundary is a convention. FDA treats any chain over 40 amino acids as a protein, which is why insulin, at 51, is regulated as a biologic.
Are peptides steroids?
No. Steroids such as testosterone are fat-soluble molecules made from cholesterol, built on a four-ring carbon skeleton. Peptides are chains of amino acids. Steroids pass into cells and bind receptors inside, while most peptides bind receptors on the cell surface. Some peptide hormones signal glands to make steroid hormones, but they are not steroids themselves.
Can peptides be taken by mouth?
Most are absorbed poorly, because digestive enzymes break peptide bonds and the gut wall blocks large molecules. Oral semaglutide is the best-known exception. It uses an absorption enhancer called SNAC, and even then its label puts bioavailability at about 2% or less.
What is the difference between mg and mcg on a peptide label?
Both are units of mass. One milligram (mg) equals 1,000 micrograms (mcg), so mixing them up creates a thousandfold error. Check which unit a label, prescription or study uses before doing any math. The mcg to mg converter handles the decimal shift.
Why is insulin measured in units instead of milligrams?
A unit measures insulin's biological effect against a reference standard rather than its weight. Concentrations are written the same way: U-100 insulin has 100 units per mL. That is why insulin syringes are marked in units. The guide to reading an insulin syringe explains the markings.
Sources and References
- National Human Genome Research Institute. Peptide (Talking Glossary of Genomic and Genetic Terms)
- IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. Nomenclature and Symbolism for Amino Acids and Peptides: peptides
- US Food and Drug Administration. FDA works to ensure smooth regulatory transition of insulin and other biological products
- Signal Transduction and Targeted Therapy (via PubMed Central). Therapeutic peptides: current applications and future directions (Wang et al., 2022)
- US Food and Drug Administration / Novo Nordisk. RYBELSUS (semaglutide) tablets prescribing information
- The Nobel Prize. The Nobel Prize in Chemistry 1984: press release
- US Food and Drug Administration. Warning letter 721806 to an online peptide seller (March 31, 2026)