Home / What are peptides? Types, examples and a starter A–Z list
Prepared by: Research Desk. Sources are linked for verification.
Definition. A peptide is a short chain of amino acids joined by peptide bonds. Chains of roughly 2 to 50 amino acids are usually called peptides. Longer chains, often folded into complex three-dimensional shapes, are called proteins. The boundary is a convention, not a sharp line.
This entry is the foundation for the Peptide Infinity encyclopedia. It covers peptide chemistry, the main types of peptides and how peptide medicines reach the market. It ends with three structured starter entries that show the format every entry follows.
Contents
1. Chemistry in brief
Amino acids and peptide bonds
The body uses 20 standard amino acids. Each has the same backbone (an amino group, a carboxyl group and a central carbon atom) plus a distinctive side chain. When the carboxyl group of one amino acid reacts with the amino group of the next, they form a peptide bond and release a molecule of water. Repeating this produces a chain with a direction: it runs from the N-terminus to the C-terminus.
Naming conventions
Peptides are often named by length:
Dipeptide: 2 amino acids
Tripeptide: 3 (for example GHK, glycyl-histidyl-lysine)
Pentapeptide: 5 (for example KTTKS, used in some cosmetics)
Pentadecapeptide: 15 (for example BPC-157)
Polypeptide: a general term for longer chains
Amino acids are often abbreviated with single letters: G for glycine, H for histidine, K for lysine, and so on. That’s why many peptide names look like strings of capital letters.
Modifications
Peptide drugs are frequently modified to make them more stable or longer-lasting. Common modifications include:
Fatty-acid chains (lipidation). These help the peptide bind to albumin in the blood so it lasts longer.
Unnatural amino acids. These resist enzymes that would otherwise break the peptide down.
Acetylation or amidation of the ends. This blocks degradation.
Cyclization. The chain is joined into a ring.
2. Types of peptides
Peptides can be classified in several overlapping ways.
By origin
Endogenous peptides are made naturally in the body. Examples include insulin, glucagon, oxytocin, GLP-1 and ghrelin.
Synthetic analogues are engineered versions of natural peptides, such as long-acting GLP-1 receptor agonists or growth-hormone-releasing hormone (GHRH) analogues.
Novel synthetic peptides have no exact natural counterpart.
By function
Hormones. Peptides that act as chemical messengers in the blood, such as insulin and GHRH.
Neuropeptides. Peptides that act in the nervous system.
Antimicrobial peptides. Part of the innate immune system.
Signal and carrier peptides (cosmetic). Used in skincare to influence the skin’s matrix or to deliver trace metals.
By regulatory use (Peptide Infinity’s main axis)
Approved medicines, such as tesamorelin for HIV-associated abdominal fat [1][2].
Diagnostic agents, such as sermorelin’s historical use as a test for growth hormone deficiency [3].
Investigational drugs in clinical trials.
Cosmetic ingredients, which are regulated as cosmetics, not drugs.
Unapproved research chemicals, with no recognized medical status.
The status explainer defines each badge in detail.
3. Why peptides make good, and difficult, medicines
Strengths:
High specificity. Peptides often bind their target receptor precisely, which can mean fewer off-target effects than small molecules.
Biological familiarity. Many mimic natural signals.
Predictable breakdown. They are metabolised into amino acids.
Challenges:
Short half-life. Natural peptides are often broken down within minutes.
Poor oral absorption. Digestive enzymes destroy most peptides, so most peptide drugs are injected. Oral peptide drugs need special formulation technology.
Manufacturing complexity. Impurities and aggregation can trigger immune reactions (immunogenicity). This is a recurring theme in FDA’s assessments of compounded peptides.
4. How a peptide becomes an approved medicine
Discovery and preclinical testing, in cells and animals.
Phase 1 trials in a small number of people, mainly to assess safety.
Phase 2 trials to find doses and look for signs that the drug works.
Phase 3 trials: large, randomized studies to confirm efficacy and characterise risks.
Regulatory review. In the U.S., this means a New Drug Application or Biologics License Application to the FDA.
Post-marketing surveillance.
Tesamorelin shows the full journey. A 26-week randomized trial in 412 people with HIV and abdominal fat accumulation found that visceral fat fell by 15.2% with tesamorelin and rose by 5.0% with placebo [4]. The drug is now FDA-approved for that specific use [1].
5. Starter entries
Each Peptide Infinity entry follows the template below. These three entries are abridged. Full entries include additional sections on pharmacology and a complete source list.
Sermorelin
Status: Approved – discontinued (U.S.). Checked 18 Oct 2026.
Identity: A 29-amino-acid analogue of human growth hormone-releasing hormone (GHRH) [3].
Mechanism: Stimulates the anterior pituitary gland to secrete growth hormone [3].
Regulatory history: It was FDA-approved as Geref (NDA 019863), which is now listed as discontinued. FDA’s Federal Register determination states that it “was not discontinued or withdrawn for safety or effectiveness reasons” [5].
Key evidence: A 1999 review described sermorelin as a relatively specific test for diagnosing growth hormone deficiency. It also cited limited data showing it increased height velocity in some prepubertal children with idiopathic growth hormone deficiency [3]. Increases in height velocity were smaller than with somatropin (recombinant growth hormone) [3].
Safety notes from the literature: Transient facial flushing and injection-site pain were the most commonly reported adverse events [3].
What’s not established: Use in adults for “anti-aging”, body composition or performance isn’t supported by FDA approval.
Tesamorelin
Status: Approved (U.S.) for a specific indication. Checked 18 Oct 2026.
Identity: A synthetic analogue of GHRH [4].
Approved product: Egrifta (tesamorelin acetate), application 022505, prescription only [1].
Approved use: Reduction of excess abdominal fat in adults with HIV-associated lipodystrophy. Described in 2024 as the only FDA-approved therapy for abdominal fat accumulation in people with HIV [2].
Key evidence:
In a randomized trial of 412 participants, visceral fat fell 15.2% with tesamorelin versus a 5.0% increase with placebo over 26 weeks. Triglycerides and the cholesterol ratio improved, and IGF-I rose by 81.0% [4].
Pooled phase 3 analyses found that participants whose visceral fat fell by at least 8% (“responders”) had better triglyceride levels and preserved glucose control [6].
In a 2024 analysis of people on modern integrase-inhibitor HIV regimens, tesamorelin reduced visceral and liver fat and was well tolerated [2].
Safety notes from the literature: In the pivotal trial, overall adverse events didn’t differ significantly between groups. More participants on tesamorelin withdrew because of an adverse event [4]. The product label lists full warnings.
What’s not established: Use for general weight loss or “anti-aging” is off-label and not supported by the approval.
Ipamorelin
Status: No FDA-approved product. Checked 18 Oct 2026.
Identity: A synthetic pentapeptide described as a selective growth hormone secretagogue and ghrelin-receptor agonist [7].
Mechanism: Activates the ghrelin receptor, which stimulates growth hormone release and gastrointestinal motility [7].
Key evidence: In a rodent model of postoperative ileus (a temporary gut paralysis after surgery), repeated ipamorelin administration increased fecal output, food intake and weight gain after surgery compared with vehicle [7]. Peptide Infinity hasn’t yet completed a verified review of human trial publications for ipamorelin. This entry will be expanded once that review is done.
Safety notes: No FDA-approved labelling exists. Products sold online are unregulated.
What’s not established: Claims about muscle building, fat loss, sleep or anti-aging in humans.
6. How to use this encyclopedia
Start with the status badge. It tells you whether a peptide has any recognized medical use.
Check the date. Regulatory status changes.
Look at the evidence type. A rodent study and a phase 3 trial don’t carry the same weight.
Use the sources. Every entry links to PubMed, ClinicalTrials.gov or FDA records.
Peptide Infinity does not provide dosing information or purchasing guidance, and it is not medical advice.
7. Common misconceptions
“Peptides are all natural, so they’re safe.” Many peptide drugs are heavily engineered synthetic molecules, and even natural peptides can cause harm at the wrong dose or with impurities. Safety is established by trials, not by origin.
“Peptides are just supplements.” Peptide drugs are regulated as drugs. Oral collagen peptides are sold as dietary supplements. Injectable peptides are not supplements.
“If it’s sold online, it must be legal.” “Research use only” labels don’t make a product approved for human use. See the status explainer.
“Approved means approved for everything.” Approval covers specific products and specific uses. Tesamorelin’s approval, for example, is for HIV-associated abdominal fat [1][2], not general weight loss.
“Growth hormone secretagogues and growth hormone are the same.” Sermorelin and tesamorelin stimulate the body’s own growth hormone release [3][4]. That’s different from administering recombinant growth hormone, and the two were not equivalent in pediatric growth studies [3].
Entry last updated: 18 October 2026.
Sources
Not medical advice. This page reports published research and regulatory records for information only.