What are peptides? Peptides vs proteins, how they are classed, and legal status

Peptides are short chains of amino acids joined by peptide (amide) bonds. The US Food and Drug Administration treats chains of 40 or fewer amino acids as peptides and longer ones as proteins (FDA). More than 80 peptide drugs have reached the market (Muttenthaler 2021), backed by approved labels and human RCTs. Most peptides sold online as “research” products are not FDA-approved, and their evidence is mostly animal or lab data. Peptides are not steroids.

Item	Status
Definition	Amino acids linked by amide bonds (IUPAC-IUB)
Peptide vs protein (US)	40 or fewer amino acids = peptide; more than 40 = protein (FDA)
US FDA status	Depends on the molecule: semaglutide (label) and bremelanotide (label) are approved; BPC-157 is not (USADA)
Compounding (503A/503B)	16 peptides “withdrawn by nominators” from Category 2; kisspeptin-10, GHRP-2, GHRP-6 and ipamorelin still in Category 2 (FDA, updated 2026-04-22)
WADA 2026	Peptide hormones: S2; non-approved peptides such as BPC-157: S0; prohibited at all times (USADA)
Strongest evidence	Approved labels and phase 3 RCTs for approved peptide drugs; mostly animal and lab data for unapproved ones
Human trials (examples)	NCT03548935, NCT04184622 (phase 3, completed); BPC-157: 1 clinical study among 36 (Vasireddi 2025)
Steroids?	No: steroids are made from cholesterol, peptides from amino acids (Miller 2011)
Last checked	2026-09-30

What changed (October 2026): a table of common peptides by US legal class is added, with the 503A and 503B compounding lists kept apart; the reported outcome of the July 2026 advisory votes and the 2027 WADA List change for peptides are added; FDA’s pages were re-checked on 2026-10-26.

What are peptides?

A peptide is “any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another”, and those amide links “may be called peptide bonds”, according to the international nomenclature rules for biochemistry (IUPAC-IUB 1984). Each amino acid in the chain is called a residue. By convention the sequence is written from the N-terminal residue (the end with a free amino group) on the left to the C-terminal residue (the end with a free carboxyl group) on the right. The same rules say peptides with “fewer than about 10–20 residues” may be called oligopeptides, longer ones polypeptides, and that polypeptides of defined sequence with “more than about 50 residues are usually known as proteins”. They also note that authors differ on where the protein label begins.

The body makes many peptides as signaling molecules. Insulin is the best-known example: in 1922, Banting, Best and colleagues reported the first use of pancreatic extracts to treat people with diabetes (Banting 1922), and in 1955 Sanger’s group described the disulphide bonds that link insulin’s chains (Ryle 1955). Glucagon-like peptide-1 (GLP-1), a gut hormone, is the template for semaglutide, which the FDA label describes as a GLP-1 analogue with 94% sequence homology to the human hormone (Rybelsus label).

Peptides also come from other species. In 1992, researchers isolated exendin-4 from the venom of the Gila monster (Heloderma suspectum) and showed that it acts on receptors in pancreatic tissue [Lab] (Eng 1992). A 2021 review in Nature Reviews Drug Discovery lists human hormones, rational chemical design and natural sources such as venoms as the main routes to peptide drugs (Muttenthaler 2021).

In the laboratory, peptides are made by chemical synthesis or by biological (recombinant) methods (Wang 2022). The chemical route traces back to 1963, when Merrifield built a four-residue peptide on a solid resin support, the method now called solid-phase peptide synthesis (Merrifield 1963).

Peptides vs proteins: what is the difference?

Chemically, peptides and proteins are the same kind of molecule, amino-acid chains, and the difference is mainly size. International nomenclature usage reserves the word protein for defined chains of more than about 50 residues and admits that the boundary is partly a matter of convention (IUPAC-IUB 1984). In US law the line is sharper, because it decides which approval pathway a medicine follows.

Feature	Peptide	Protein
IUPAC usage	Up to about 50 residues	More than about 50 residues
FDA definition	40 or fewer amino acids	More than 40 amino acids
US pathway	Drug (new drug application)	Biologic (license) since 2020-03-23
Examples	Semaglutide, bremelanotide	Insulin, human growth hormone

The FDA’s 2020 final rule defines a protein as “any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size”. When two or more chains are joined as they are in nature, their amino acids are counted together. Under that rule insulin, with chains of 21 and 30 amino acids, counts as a 51-amino-acid protein (Federal Register 2020).

On March 23, 2020, approved applications for protein products that had historically been approved as drugs, including insulins, human growth hormone and pancreatic enzymes, were “deemed to be” biologics licenses. The FDA states that this change “does not affect ‘peptides’ (i.e., polymers composed of 40 or fewer amino acids)”, which stay regulated as drugs (FDA). The practical result: a molecule called a “peptide” in popular usage, such as human growth hormone, is not always a peptide in regulatory terms.

How are peptides classified?

No single official classification exists. Researchers and regulators group peptides in at least five overlapping ways: by length, origin, biological target, chemical structure and regulatory status. This site groups its entries by target and mechanism.

Basis	Classes	Examples
Length	Dipeptide, tripeptide, oligopeptide, polypeptide	KPV (3 residues); bremelanotide (7)
Origin	Human hormones, venom-derived, synthetic analogues	Insulin; exendin-4; semaglutide
Target	Incretin, melanocortin, GH secretagogue, others	Tirzepatide; bremelanotide; ipamorelin
Structure	Linear, cyclic, lipidated	Cyclic: bremelanotide; fatty-acid chain: tirzepatide
Regulatory	Approved drug, compounded, unapproved	Vyleesi; Category 2 list; “research” products

By length: the IUPAC-IUB terms di-, tri-, oligo- and polypeptide describe residue counts, with no hard cut-offs below the protein range. The KPV tripeptide is three residues long; the FDA label describes bremelanotide as a synthetic cyclic heptapeptide, seven residues (Vyleesi label).

By target: incretin peptides act on GLP-1, GIP and related receptors; melanocortin peptides act on melanocortin receptors; growth hormone secretagogues act on the growth hormone axis. Other groups covered on this site include copper-binding peptides such as GHK-Cu, mitochondria-derived peptides such as MOTS-c, and tissue-repair peptides such as BPC-157 and TB-500; each entry grades its own evidence.

By structure: chemists modify natural sequences to slow their breakdown and extend their action. Wang and colleagues describe design and delivery strategies that “overcome the inherent drawbacks of peptides” (Wang 2022). Tirzepatide, for example, carries two non-coded amino acids and a fatty-acid side chain that binds albumin (Mounjaro label).

By regulatory status: the same word “peptide” covers approved prescription drugs, substances nominated for pharmacy compounding, cosmetic ingredients and unapproved products sold online. For compounding, the FDA sorts nominated bulk substances into categories, one of which is reserved for substances with “significant safety risks”, and several peptides appear in it (FDA 503A categories).

Common peptides by US legal class, as of October 2026:

Legal class	What it means	Examples
FDA-approved drug	Prescription use for the labeled condition	Semaglutide, tirzepatide, bremelanotide, tesamorelin
Investigational	Trials or expanded access only; cannot be compounded under federal law	Retatrutide (FDA)
503A Category 1	FDA does not intend to act against compounding with it, under conditions; not an approval	GHK-Cu (non-injectable), glutathione (FDA list)
503A Category 2	Significant safety risks identified; outside that policy	Kisspeptin-10
503B Category 2	The same, for outsourcing facilities	GHRP-2, GHRP-6, ipamorelin acetate (FDA)
In no category	Nominations withdrawn; unapproved drugs	BPC-157, TB-500, CJC-1295
Cosmetic ingredient	Used in skin products, not sold as a drug	Copper tripeptide-1, palmitoyl pentapeptide-4

Are peptides steroids?

No. Steroid hormones are made in the body from cholesterol through a chain of enzyme steps called steroidogenesis (Miller and Auchus 2011). Peptides are chains of amino acids. The two groups differ in chemistry, in how they are regulated and in how anti-doping rules classify them. They are often mentioned together because both are misused as performance-enhancing drugs: an Endocrine Society scientific statement reviews anabolic steroids and peptide hormones side by side and links performance-enhancing drug use to cardiovascular, psychiatric, hepatic, endocrine and other disorders [Human observational] (Pope 2014).

Feature	Peptides	Anabolic steroids
Built from	Amino acids	Cholesterol
US legal class	Drug, biologic or unapproved product	Schedule III controlled substances
WADA class	S2 (peptide hormones) or S0 (non-approved)	S1 (anabolic agents)

In the US, anabolic steroids are Schedule III controlled substances under the Controlled Substances Act, and the Drug Enforcement Administration adds new steroids to that schedule by rule (DEA 2009). In sport, anabolic steroids such as boldenone sit in WADA class S1, anabolic agents, prohibited at all times (USADA). Peptide hormones and growth factors sit in a separate class, S2, and experimental peptides without any government approval, such as BPC-157, fall under S0 (USADA).

How do peptides work in the body?

• Receptor binding: many peptide drugs work as receptor agonists. Semaglutide is a GLP-1 receptor agonist [Approved label] (Rybelsus label); bremelanotide activates several melanocortin receptor subtypes [Approved label] (Vyleesi label).
• Engineered half-life: modified peptides are built to last. The labeled elimination half-life is about one week for semaglutide (Rybelsus label) and about five days for tirzepatide [Approved label] (Mounjaro label).
• Poor oral absorption: oral semaglutide needs the absorption enhancer salcaprozate sodium (SNAC), is absorbed mainly in the stomach and still reaches only about 0.4% to 1% bioavailability [Approved label] (Rybelsus label).
• Multiple targets: newer peptides are built to act on more than one receptor. Retatrutide, an investigational triple-hormone-receptor agonist, acts on GIP, GLP-1 and glucagon receptors; its first phase 3 results are in the tables below [Human RCT, phase 3] (Jastreboff 2026).

What do peptides do? Benefits the research shows

As medicines, peptides deliver some of the largest benefits in modern drug trials. Once-weekly semaglutide reduced body weight by 14.9% versus 2.4% with placebo over 68 weeks [Human RCT, n=1,961] (Wilding 2021), tirzepatide by up to 20.9% versus 3.1% over 72 weeks [Human RCT, n=2,539] (Jastreboff 2022), and the investigational drug retatrutide by 25.0% on its highest dose versus 3.9% over 80 weeks in phase 3 [Human RCT, n=2,339] (Jastreboff 2026). Bremelanotide is approved to treat low sexual desire in women [Approved label] (Vyleesi label). For research peptides sold online, most benefits have so far been shown in animals.

“Peptides” is too broad a word for a single verdict. The strength of the evidence depends entirely on the molecule. The table grades common claims made about peptides as a group.

Claim	Evidence level	Best study and result
A GLP-1 peptide reduces body weight	Human RCT (n=1,961)	Wilding 2021: −14.9% vs −2.4% placebo at 68 weeks
A GIP/GLP-1 peptide reduces body weight	Human RCT (n=2,539)	Jastreboff 2022: up to −20.9% vs −3.1% at 72 weeks
Retatrutide reduces body weight	Human RCT, phase 3 (n=2,339)	Jastreboff 2026 (TRIUMPH-1): −25.0% on 12 mg vs −3.9% placebo at 80 weeks; not approved
Bremelanotide treats low sexual desire in women	Approved label	Vyleesi label: approved for acquired, generalized HSDD
Collagen peptides reverse skin aging	Human RCT meta-analysis (23 RCTs)	Myung 2025: no effect in non-industry or high-quality trials
BPC-157 heals tendons and muscle	Animal (35 of 36 studies)	Vasireddi 2025: one retrospective human study
Peptides work well as pills	Approved label (against)	Rybelsus label: 0.4–1% oral bioavailability
Venom peptides act on mammalian receptors	Lab (in vitro)	Eng 1992: exendin-4 acts on pancreatic tissue receptors

Approved metabolic peptides. The strongest peptide evidence comes from large phase 3 trials of GLP-1-based drugs. In STEP 1, once-weekly semaglutide reduced body weight by 14.9% versus 2.4% with placebo over 68 weeks, and 86.4% of participants lost at least 5% of body weight versus 31.5% on placebo [Human RCT, n=1,961] (Wilding 2021). In SURMOUNT-1, tirzepatide reduced body weight by 15.0% to 20.9% across three dose groups versus 3.1% with placebo over 72 weeks [Human RCT, n=2,539] (Jastreboff 2022).

Supplements and cosmetics. Oral collagen peptides are among the most widely sold peptide products. A 2025 meta-analysis of 23 randomized trials found improvements in skin hydration, elasticity and wrinkles when all trials were pooled. Trials without pharmaceutical-company funding showed no effect, nor did high-quality trials, and the authors concluded there is “no clinical evidence to support the use of collagen supplements to prevent or treat skin aging” [Human RCT meta-analysis] (Myung 2025).

Unapproved peptides sold online. For the peptides most discussed online, the evidence is mostly preclinical. A 2025 systematic review of BPC-157 in orthopaedic sports medicine found 36 studies, of which 35 were preclinical and 1 was a retrospective clinical study [Animal] (Vasireddi 2025). A 2026 narrative review describes the peptides promoted in online “biohacking” spaces as “experimental or weakly evidenced” (Hailu 2026).

Human studies and trials

The three completed trials below underpin the best-known modern peptide results. Each was confirmed as completed on ClinicalTrials.gov on 2026-09-30.

Trial	Phase, n, status	Result
STEP 1, semaglutide (NCT03548935)	Phase 3, n=1,961, completed	−14.9% vs −2.4% weight at 68 weeks
SURMOUNT-1, tirzepatide (NCT04184622)	Phase 3, n=2,539, completed	Up to −20.9% vs −3.1% at 72 weeks
TRIUMPH-1, retatrutide (NCT05929066)	Phase 3, n=2,339 randomized, completed	−25.0% (12 mg) vs −3.9% at 80 weeks

Retatrutide remains investigational and is not approved for any use; the FDA states that it cannot be used in compounding under federal law (FDA). Its entry is at retatrutide (LY3437943). For BPC-157, one of the most discussed “research” peptides, the systematic review above found a single retrospective human study and no controlled trial (Vasireddi 2025).

Side effects and safety

Approved peptide drugs. Gastrointestinal effects dominate. Nausea and diarrhea were the most common adverse events with semaglutide in STEP 1 [Human RCT, n=1,961] (Wilding 2021), and gastrointestinal events with tirzepatide occurred mainly while the dose was being increased [Human RCT, n=2,539] (Jastreboff 2022). Tirzepatide carries a boxed warning because it caused thyroid C-cell tumors in rats; whether it does so in humans is unknown [Approved label] (Mounjaro label). Semaglutide carries a similar boxed warning based on rodent data, and semaglutide tablets are contraindicated in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 [Approved label] (Rybelsus label). With bremelanotide, nausea affected 40% of treated patients in trials [Approved label] (Vyleesi label).

Immunogenicity and impurities. Immunogenicity means the risk that the immune system reacts to a substance. Explaining why BPC-157 was placed in Category 2, the FDA wrote that compounded BPC-157 “may pose risk for immunogenicity for certain routes of administration and may have complexities with regard to peptide-related impurities and active pharmaceutical ingredient (API) characterization” (FDA).

Unregulated and compounded products. As of May 2026, the FDA had received nearly 1,000 adverse event reports for compounded semaglutide and more than 730 for compounded tirzepatide, and it reports hospitalizations linked to administration and calculation errors (FDA). Australia’s regulator warns that for unapproved peptide products it is unknown how they are manufactured, whether they are sterile, what side effects they may cause and what they actually contain (TGA). A 2026 review frames the core risk as “products of uncertain identity, purity, potency, sterility, and safety” (Hailu 2026).

Who should not use them. For approved peptide drugs, the label lists contraindications, such as the thyroid-cancer history noted above. Competitive athletes face anti-doping sanctions: USADA states that topical use of a prohibited peptide or growth factor is as prohibited as any other route (USADA).

Legal and regulatory status

There is no single legal status for “peptides”. Each molecule has its own, and status also depends on where a product is sold and how it is labeled. The table summarizes the rules that apply to the category as a whole, checked on 2026-09-30; the compounding and sport rows were re-checked on 2026-10-26.

Jurisdiction	Rule or status	Source
US FDA	40 or fewer amino acids: drug; more than 40: biologic	FDA
US compounding	16 peptides withdrawn from Category 2; 4 remain: kisspeptin-10 (503A), GHRP-2, GHRP-6 and ipamorelin (503B)	FDA, 2026-04-22
US advisory review	7 peptides reviewed for the 503A bulks list, 2026-07-23/24; no proposed rule by 2026-10-26	FDA PCAC
US “research use only”	Warning letters 2026-08-24: labels did not change drug status	FDA
WADA 2026	S2 peptide hormones; S0 non-approved; S1 steroids	USADA
WADA 2027	From 2027-01-01, S0 names peptides in general	USADA
UK	MHRA seized unlicensed weight-loss drugs and peptide products, 2026-05-28	MHRA
Australia	Unapproved peptides not assessed by the TGA; advertising likely unlawful	TGA

Compounding categories. For pharmacy compounding under section 503A, the FDA sorts nominated bulk substances into three interim categories. Category 1 substances “may be eligible for inclusion on the 503A bulks list”; Category 2 substances also may be eligible, “however, FDA has identified significant safety risks”; Category 3 substances were nominated with insufficient information. The agency does not intend to categorize substances nominated on or after January 7, 2025 (FDA).

The 2026 Category 2 changes. The FDA’s Category 2 page, updated 2026-04-22, lists 16 substances as “previously in Category 2 – withdrawn by nominators”, including BPC-157, TB-500, GHK-Cu, KPV, MOTS-c, CJC-1295, Melanotan II, semax, selank, epitalon and thymosin alpha-1. Kisspeptin-10 remains in Category 2 for 503A compounding, and growth hormone releasing peptide-2, growth hormone releasing peptide-6 and ipamorelin acetate remain in Category 2 for 503B outsourcing facilities (FDA). Withdrawal from Category 2 is not an FDA approval, and the FDA lists these peptides as withdrawn, not as Category 1.

Advisory committee review. On July 23–24, 2026, the FDA’s Pharmacy Compounding Advisory Committee reviewed BPC-157 (for ulcerative colitis), KPV, TB-500, MOTS-c, emideltide (DSIP), semax and epitalon for possible inclusion on the 503A bulks list (FDA). Advisory committees advise the FDA; the meeting did not itself add any substance to the list. Trade press reported votes in favor of six of the seven, all but emideltide; when checked on 2026-10-26, FDA had posted no minutes and the Federal Register showed no proposed rule.

“Research use only” products. In warning letters dated August 24, 2026, the FDA found that website content showed products labeled “research use only” and “not for human consumption” were “intended to be drugs for human use”. The products named in one letter included semaglutide, retatrutide, SS-31, PT-141 and tesamorelin (FDA warning letter).

Sport. The 2026 WADA Prohibited List keeps peptide hormones and growth factors in S2 and added pegmolesatide as a new S2 example; athletes are “strictly liable” for what is in their systems (USADA). BPC-157 is prohibited under S0, non-approved substances (USADA). Most cosmetic peptides, such as palmitoyl pentapeptide-4 and non-palmitoylated GHK, are permitted (USADA). From 1 January 2027, the S0 class names “peptides” in general as prohibited non-approved substances (USADA). A peptide-by-peptide list is at WADA banned peptides.

UK and Australia. On May 28, 2026, the UK Medicines and Healthcare products Regulatory Agency (MHRA) seized about 12,000 doses of unlicensed weight-loss medicines, including retatrutide and tirzepatide, together with peptide products (MHRA). Australia’s Therapeutic Goods Administration states that unapproved peptide products have not been assessed for safety, quality or effectiveness, and that promoting them through social media or influencers is likely to breach advertising law (TGA).