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The basics

GLP-1 peptides: what are they, and what are "research peptides"? What the FDA says

Semaglutide, tirzepatide and liraglutide are peptides, which is why most of them are injected. Here is what that means in plain English, how the newer pills get around it, and what the FDA and independent testing say about "research peptides" sold online.

By 8 min read
Illustration of a short chain of beads representing a peptide.
The short version
  • GLP-1 peptides are medicines made of a short chain of amino acids: semaglutide, tirzepatide and liraglutide are all peptides, with molecular weights of about 3,750 to 4,810 on their US labels. Orforglipron (Foundayo) is not a peptide.1,2,3,4
  • Being a peptide is why most GLP-1s are injected: semaglutide is 89% absorbed from an injection but only about 1% to 2% from a tablet, according to the Wegovy label.1
  • "Research peptides" sold online are not approved medicines. The FDA calls them illegal unapproved drugs, and testing has found low purity and contamination.5,12

What is a peptide?

A peptide is a short chain of amino acids, the same building blocks that make up proteins. Proteins are long chains; peptides are short ones. Many hormones in your body are peptides, including GLP-1 (glucagon-like peptide-1), a gut hormone released after you eat that helps control blood sugar and appetite. For a primer on the hormone itself, see what is GLP-1.

Your own GLP-1 is quickly broken down by an enzyme called DPP-4. The medicines are modified versions that last much longer. The Wegovy label, for example, says semaglutide has a change at one position to protect it from DPP-4, and a fatty acid chain that helps it bind to albumin, a blood protein, so it stays in the body.1

Illustration of a plain vial beside a dotted question mark.

Which GLP-1 medicines are peptides?

Medicine (brands)Peptide?Molecular weight (label)Absorption (label)
Semaglutide (Ozempic, Wegovy, Rybelsus)Yes4113.58 g/mol89% from an injection; about 1% to 2% from a Wegovy tablet
Tirzepatide (Mounjaro, Zepbound)Yes4813.53 Da80% from an injection
Liraglutide (Victoza, Saxenda)Yes3751.2 DaAbout 55% from an injection
Orforglipron (Foundayo)No, small molecule902.0 g/mol (calcium salt)77% from a tablet, after a 0.8 mg dose
 Source: US prescribing information, sections 11 (Description) and 12.3 (Pharmacokinetics), for Wegovy, Zepbound, Saxenda and Foundayo.1,2,3,4

The Saxenda label says liraglutide is 97% the same as human GLP-1.3 Tirzepatide is different: its label says it is built on the sequence of a second gut hormone, GIP, and it acts on both GIP and GLP-1 receptors.2 For more on each, see what is semaglutide and what is tirzepatide.

Why does it matter whether a GLP-1 is a peptide?

Peptides are large, fragile molecules. Swallowed, most of a peptide is broken down by stomach acid and digestive enzymes before it can be absorbed, much like the protein in food. That is why insulin, semaglutide and tirzepatide have mostly been injections.

89%of injected semaglutide reaches the bloodstream (Wegovy label)
~1–2%of semaglutide from a Wegovy tablet reaches the bloodstream (Wegovy label)

Semaglutide tablets get around this with an absorption helper called SNAC, mixed into the tablet. Even so, the Wegovy label estimates only about 1% to 2% is absorbed, which is why the tablet doses are much higher in milligrams than the injection doses. The label instructs taking the tablet on an empty stomach with up to 4 ounces of water, then waiting at least 30 minutes before eating, drinking or taking other oral medicines.1

Orforglipron is a small molecule, closer to a conventional pill. Its label reports 77% absorption after a 0.8 mg dose and instructs that it can be taken with or without food.4 For a side-by-side look at the tablets, see GLP-1 pills and orforglipron weight loss.

Peptides also need careful manufacturing. Small changes in a peptide's structure, or impurities left over from making it, can change how it works or trigger an immune reaction. The FDA has described these challenges in its work on generic peptide medicines.6 That matters for the next section.

What are "research peptides"?

"Research peptides" are powders or liquids sold online, often labeled "for research use only" or "not for human consumption". Many sellers list semaglutide and tirzepatide, but also substances that are not FDA-approved for any use, such as:

  • Retatrutide, an experimental medicine that acts on GIP, GLP-1 and glucagon receptors.14 Some sellers call it "GLP-3", which is not a real drug class.9 See retatrutide weight loss for what its trials found.
  • Cagrilintide, an experimental Novo Nordisk medicine that the company lists as in phase 3 trials for obesity.7
  • Other peptides such as mazdutide and survodutide, which are not FDA-approved.8,9

The FDA says retatrutide and cagrilintide are not components of any FDA-approved drug, have not been found safe and effective for any condition, and cannot be used in compounding under federal law.5

Is "research use only" legal? What the FDA says

The FDA says companies that sell unapproved GLP-1 drugs labeled "for research purposes" or "not for human consumption" directly to consumers, with dosing instructions, are selling them illegally. It urges consumers not to buy them, because they are of unknown quality and may be harmful.5

FDA warning letterProducts named (as listed)What the FDA said
Swisschems, December 10, 2024Retatrutide and other GLP-1 productsCalled its retatrutide an unapproved new drug despite a "research chemicals" label10
Prime Sciences, March 31, 2026Cagrilintide, "GLP1-R" (retatrutide), "GLP1-S" (semaglutide), "GLP1-T" (tirzepatide), mazdutide, a reconstitution kit"Laboratory research purposes only" labeling did not change that the website showed the products were meant for people8
NuScience Peptides, August 24, 2026"GLP-3 RT (Retatrutide) Research Peptide", "GLP-1 Sema", "GLP-2 Tirz", survodutide, mazdutide, othersWebsite evidence showed the products were intended as drugs for human use9
 Source: FDA warning letters posted on fda.gov. These are examples, not a full list.

In these letters the FDA pointed to what the websites said, such as weight-loss claims and how the products work, rather than the disclaimer on the label. It also treated bacteriostatic water sold alongside the peptides, plus dosing "calculators", as signs the products were meant to be injected by people. Companies were given 15 working days to respond.8,9

Separately, the FDA keeps a list of bulk substances it says may pose significant safety risks in compounding. As of April 22, 2026, it included several non-GLP-1 peptides such as GHRP-2, GHRP-6, ipamorelin acetate and kisspeptin-10, with immune reactions and limited safety data among the reasons given.11

What did independent testing find?

Few products have been tested, and the studies are small or early. Here is what has been published.

  • Semaglutide from illegal online sellers (J Med Internet Res, 2024). Researchers made test purchases from 6 websites selling semaglutide without a prescription. Only 3 vials arrived; the 3 Ozempic pens ordered never came. The vials held 28.56% to 38.69% more semaglutide than labeled, but purity was only 7.7% to 14.37%, against the 99% claimed. Endotoxin, a marker of bacterial contamination, was found in all samples.12
  • A large testing dataset (preprint, 2026). This study has not been peer reviewed. Authors analyzed 6,441 samples of 14 gray-market peptides, including retatrutide, semaglutide, tirzepatide and cagrilintide, from a public independent-testing database. Depending on the quality standard used, 41.6% to 71.1% failed basic quality criteria, and 15% had measurable endotoxin.13
  • What people report on Reddit (JAMA Network Open, 2026). This observational study used AI tools to read posts from 13,589 Reddit users who described taking retatrutide. Among 7,823 users with a mapped symptom, the most common were fatigue (15.2%), nausea (11.6%), insomnia (6.1%) and increased heart rate (4.8%). The authors say product identity, purity and dose could not be checked, so the study cannot show how often side effects happen or what caused them.14
  • Preparing a dose (Public Health Reports, 2026). In a small test, 20 adults with no injection training tried to prepare and inject gray-market semaglutide using only online information. On average they completed 15 of 34 steps correctly, with errors that could cause wrong doses, needle-stick injuries or infections.15
None of the trial results for semaglutide, tirzepatide or retatrutide apply to research peptides, because nobody has checked what is in a given vial. If you are considering one, please talk to a licensed prescriber first. If you have used one and feel unwell, seek medical care and tell the clinician exactly what you took. The FDA asks people to report problems to its MedWatch program.5

How are research peptides different from compounded GLP-1s?

They are not the same thing. Compounded semaglutide or tirzepatide is made by a state-licensed pharmacy for a prescribed patient. It is not FDA-approved either, and the FDA has its own concerns about it, which we cover in compounded semaglutide and tirzepatide. Research peptides skip the pharmacy and the prescription entirely. Approved brand-name products, by contrast, were tested in large trials and are made to FDA-reviewed standards. For a list of the approved options, see GLP-1 drugs list.

Common questions

What are GLP-1 peptides?

GLP-1 peptides are medicines built from a short chain of amino acids that copy or adapt the body's own GLP-1 hormone. Semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound) and liraglutide (Victoza, Saxenda) are all peptides; semaglutide's US label gives its molecular weight as 4113.58 g/mol.

Is orforglipron a peptide?

No. Orforglipron (Foundayo) is a small-molecule GLP-1 receptor agonist with a molecular weight of 902.0 g/mol, according to its US label. The label reports 77% absolute bioavailability after a 0.8 mg dose and says it can be taken with or without food.

Are research peptides legal?

The FDA says GLP-1 products labeled "for research purposes" or "not for human consumption" but sold to consumers are illegal unapproved drugs. In warning letters dated March 31 and August 24, 2026, it said website evidence showed the products were intended for human use despite those labels.

What is GLP-3?

GLP-3 is not an approved medicine or an official drug class. It is a name some online sellers use for retatrutide; an FDA warning letter dated August 24, 2026 lists a product called "GLP-3 RT (Retatrutide) Research Peptide". Retatrutide is still investigational and not FDA-approved for any use.

Are research peptides pure?

Testing suggests often not. In a 2024 study that bought semaglutide from illegal online sellers, the 3 vials that arrived had 7.7% to 14.37% purity instead of the 99% claimed, and endotoxin was found in all of them.

Sources

  1. US FDA. WEGOVY (semaglutide) injection and tablets, prescribing information, sections 2.1, 11 and 12.3. DailyMed, effective June 2026. DailyMed
  2. US FDA. ZEPBOUND (tirzepatide) injection, prescribing information, sections 11 and 12.3. DailyMed, effective August 2026. DailyMed
  3. US FDA. SAXENDA (liraglutide) injection, prescribing information, sections 11 and 12.3. DailyMed, effective February 2026. DailyMed
  4. US FDA. FOUNDAYO (orforglipron) tablets, prescribing information, sections 2.1, 11 and 12.3. DailyMed, effective July 2026. DailyMed
  5. US Food and Drug Administration. FDA's concerns with unapproved GLP-1 drugs used for weight loss. Content current as of October 1, 2026. fda.gov
  6. US Food and Drug Administration. Performance Report to Congress: Generic Drug User Fee Amendments, FY 2025. fda.gov (PDF)
  7. Novo Nordisk A/S. Annual report on Form 20-F for the fiscal year 2025. US Securities and Exchange Commission, 2026. sec.gov
  8. US Food and Drug Administration. Warning letter to Prime Sciences (MARCS-CMS 721805). March 31, 2026. fda.gov
  9. US Food and Drug Administration. Warning letter to NuScience Peptides LLC (MARCS-CMS 733652). August 24, 2026. fda.gov
  10. US Food and Drug Administration. Warning letter to Swisschems (MARCS-CMS 695663). December 10, 2024. fda.gov
  11. US Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Content current as of April 22, 2026. fda.gov
  12. Ashraf AR, Mackey TK, Vida RG, et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study. J Med Internet Res. 2024;26:e65440. doi:10.2196/65440 · Full text (PMC)
  13. Evaluation of Research Grade Peptides Marketed Directly to Consumers Reveals Extensive Variability in Purity and Measured Abundance. Preprints.org, April 2026. Preprint, not peer reviewed. doi:10.20944/preprints202604.1748.v1
  14. Sehgal NKR, Tronieri JS, Rader B, Ungar L, Guntuku SC. Self-Reported Adverse Effects Among Social Media Users Taking Nonapproved Retatrutide. JAMA Netw Open. 2026;9(10):e2638929. doi:10.1001/jamanetworkopen.2026.38929 · Full text (PMC)
  15. Sedensky A, Nuzi K, Nolan L, Belcourt J, White CM. Ability of Adults to Correctly Use Grey Market Peptide Semaglutide GLP-1 Receptor Agonists Acquired Without a Prescription. Public Health Rep. 2026. Online ahead of print. doi:10.1177/00333549261464876 · PubMed
This article summarises published research for general information. It is not medical advice and does not replace advice from your own care team. Never change a medication or dose without talking to your prescriber.

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