What are peptides? A chemical definition
The question “what are peptides?” has a chemical answer. Peptides are compounds made when two or more amino acids join through amide bonds. In peptide chemistry, those links are called peptide bonds. The amino acids become residues. Their order forms a defined sequence. Different sequences can have very different effects. The word peptide describes structure. It does not tell you whether one substance has strong human data, limited data, or no published human result. This definition and the source boundaries below were checked 22 August 2026.
What is a peptide bond?
An amino acid has an amino group. It also has a carboxyl group. A peptide forms when one carboxyl group makes an amide bond with another amino acid's amino group. IUPAC says that this amide link may be called a peptide bond.
The joined units are called amino-acid residues. Their order is the peptide's sequence. Glycylalanine and alanylglycine contain the same two amino acids. Their order differs. They are different peptides.
Primary chemistry source: IUPAC-IUB peptide nomenclature, section 3AA-11. The page returned HTTP 200 and was checked 22 August 2026.
What is the difference between a peptide and a protein?
There is no single worldwide residue count that every source uses. IUPAC says authors differ greatly on where they begin using the word protein.
A United States regulation defines a protein as an alpha-amino-acid polymer with more than 40 amino acids (21 CFR 600.3(h)(6)). Its sequence must be defined. The rule serves a regulatory purpose. It does not define every shorter chain as a peptide.
The current European Medicines Agency guideline gives no amino-acid threshold. It places synthetic peptides “at the interface of small molecules and proteins.” The document is EMA/CHMP/CVMP/QWP/367182/2025. It took effect on 1 June 2026.
These sources use distinct peptide-versus-protein conventions. The US rule was checked 22 August 2026. The EMA synthetic-peptide guideline was checked on the same date.
What are synthetic peptides?
Synthetic peptide describes how a peptide is made. The EMA guideline covers peptides made by chemical synthesis. It expressly excludes biological and biotechnological products made with recombinant technology.
The word synthetic does not state whether a study worked. It does not state whether a product matched its label. Manufacturing route answers one question. Human data answer another. Product tests answer a third.
What do peptides do?
Peptides do not share one effect. The studies below measured distinct outcomes for named sequences. Each result belongs to one peptide. It also belongs to a population, route, and study.
The examples below show the range. Positive results lead each entry. Limits and funding stay beside them.
| Peptide | What a human or direct study measured | Evidence limit and funding |
|---|---|---|
| Retatrutide | A Phase 3 trial enrolled 537 adults with type 2 diabetes (PMID 42250575). At 40 weeks, mean body-weight change reached -15.3% (SE 0.8) in one active group. Mean change with placebo was -2.6% (SE 0.5). | Gut events were most frequent. They were generally mild to moderate. Two deaths occurred. The authors assessed both as unrelated to the study drug. Funded by Eli Lilly and Company. Checked 22 August 2026. |
| Ipamorelin | A study enrolled 40 healthy men (PMID 10496658). It recorded growth-hormone release at all five research levels. | It measured hormones. It did not measure muscle gain, recovery, sleep, or fat loss. The PubMed record lists no grant. Checked 22 August 2026. |
| BPC-157 | A single-arm report included 12 women (PMID 39325560). 10 of 12 reported complete resolution of bladder symptoms after one treatment. | The other two reported 80% improvement. No comparator was used. No adverse event was reported. The PubMed record lists no grant. Checked 22 August 2026. |
| TB-500 | A direct study used 32 rats (PMID 42542926). The TB-500 group had a higher maximum tendon load before failure than controls. | The exact PubMed clinical-trial search returned one unrelated tuberculosis paper, PMID 33318598. No published human result for the exact fragment was found. The rat-paper PubMed record lists no grant. Checked 22 August 2026. |
Can a registered trial have no public result?
Yes. NCT01280344 is an interventional ClinicalTrials.gov record. It describes a randomized, placebo-controlled Phase 2 ipamorelin study. The study enrolled 320 people after bowel surgery. Helsinn Therapeutics was the industry sponsor.
The study completed in May 2014. The record posts no results and links no results paper. Its summary does not call the record an example. The API returned HTTP 200 and was checked 22 August 2026.
Where can a reader go next?
The peptide guide lists the compounds with full written pages and the questions each page answers.
The compound register separates human data, agency status, and supply data. It does this for each named substance. The weight-loss hub compares measured outcomes in that field. The training and recovery hub covers compounds discussed for those goals. The tanning hub does the same for melanotan II.
The vial-testing guide explains identity, amount, purity, endotoxin, and sterility tests. A trial result and a vial test answer different questions.