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Peptides Explained

Why peptides are injected, not swallowed

In short

About 6 minutes for the short version; 11 with every detail open.

  • Your gut digests protein. Peptides are made of the same parts, so it digests them too.
  • Peptides are also too big and too water-loving to pass easily through the gut wall.
  • That is why most peptide medicines are injected under the skin.
  • The semaglutide tablet uses a helper substance. Still, only 1 or 2 in 100 parts reach the blood.
  • BPC-157 is also sold as capsules and sprays. No study in people has measured how much gets in.
Last reviewed
The lining of the stomach under a microscope, stained pink and purple, with deep folds and tightly packed gland cells.
The lining of the stomach under a microscope. Its cells release pepsin, an enzyme that cuts protein. Photo: Nephron (Wikimedia Commons) CC BY-SA 3.0 Image details

Each section starts with the short answer. The rest is folded under “In detail”.

Peptides are digested in the gut because the same enzymes that break down food protein cut peptide bonds. So most peptide medicines are injected. This page follows a swallowed peptide step by step, then looks at the exceptions: the semaglutide tablet, small-molecule pills, and the capsules and sprays sold online.

The gut is built to take protein apart

Why can't you just swallow a peptide?

Your gut digests it like food protein, then blocks most of what is left.
A swallowed peptide meets five barriers before it reaches the blood.
  1. Stomach acid. Acid can unfold a protein’s shape. The enzyme pepsin, most active in strong acid, starts cutting the chain.
  2. Pancreatic enzymes. In the small intestine, enzymes from the pancreas (trypsin, chymotrypsin, elastase, carboxypeptidases) cut it into smaller pieces.
  3. The gut lining. Enzymes on the surface of the gut cells cut the pieces down to single amino acids and pairs or triplets, which the body absorbs as food.
  4. The gut wall. Any intact peptide left is too big and too water-loving to pass through the cells, and the seals between cells (tight junctions) keep large molecules out.
  5. The liver. What does get through can be broken down in the gut wall or the liver before it reaches the rest of the body.

Sources 1, 2, 3

This is not a flaw in the peptide. It is the gut doing its job: turning protein into amino acids for food.1 Injecting under the skin skips all five barriers, which is why most peptide medicines are given that way.3,4

In detail Each barrier in more detail

The stomach

Many proteins are stable only within a narrow range of acidity and can unfold in stomach fluid. Pepsin is most active at pH 2 to 3 and stops working above pH 5.1 In laboratory tests, no insulin could be detected after 30 minutes in simulated stomach fluid .1

The small intestine

The pancreas releases a set of protein-cutting enzymes into the upper small intestine, such as trypsin, chymotrypsin, carboxypeptidase and elastase. Enzymes fixed to the surface of the gut cells (the brush border) then finish the job, cutting what is left into single amino acids and pairs or triplets.1

The gut wall

Cells of the gut lining form a continuous single layer, sealed to each other by tight junctions that make it a gatekeeper against large molecules. Large size and a strong attraction to water both make it hard for a peptide to pass through the cells’ oily membranes.1 A widely used rule of thumb from drug chemistry says that poor absorption is more likely above a molecular weight of 500; many peptide medicines weigh several thousand (see what is a peptide).5

The liver, and differences between people

Some of what is absorbed can be broken down in the gut wall or the liver before it reaches the rest of the body. This is called first-pass metabolism.2 On top of all this, the gut differs a lot between people, which makes the share that gets through hard to predict.1 The semaglutide tablet shows this: the share absorbed varies widely from person to person (see below).6

How much gets through

How much of a swallowed peptide reaches the blood?

Very little. Even special tablets get only 1 or 2 in 100 parts through.

Injected: most of it reaches the blood

89%

of the dose got into the blood after a shot under the skin

Who took part Semaglutide; EU product information for Wegovy Source Wegovy (semaglutide)7 Evidence Established

Swallowed: only a tiny share gets in

about 1–2%

of the dose got into the blood from a tablet, even with a helper

Who took part Semaglutide tablets (Rybelsus, Wegovy tablets); EU product information Sources Rybelsus (semaglutide); Wegovy (semaglutide)6,7 Evidence Established

The same molecule, swallowed instead of injected, reaches the blood about 45 to 90 times less. A review of oral peptide medicines that use absorption helpers puts typical oral bioavailability at about 1%, and variable.8

In detail Other approved oral peptides, and the natural exceptions

Desmopressin: an old oral peptide

Desmopressin is a synthetic, modified version of the hormone vasopressin, first approved in the US in 1978. Its tablets are an approved treatment, but the US label states that their bioavailability is about 0.16% of an injection into a vein, and about 5% of the nasal spray.9 A tablet can still be useful when the molecule is potent enough that a tiny share is sufficient.8

Peptides that do survive the gut

Some peptides are built differently. Cyclosporin is a ring-shaped peptide of molecular weight 1202.1 It has been approved in the US as a medicine taken by mouth since 1983, first as a drink and from 1990 also as capsules.10 Alpha-amanitin, the ring-shaped toxin of the death cap mushroom, resists acid and enzymes, so it survives digestion and damages the liver within days of a meal.11 Researchers now design small, compact ring-shaped peptides for this reason: they may be more orally available than ordinary linear chains.8 Being a peptide does not rule out oral absorption; it makes it the exception, which has to be shown for each molecule.

How the semaglutide tablet gets round the problem

How does the semaglutide tablet work, then?

A helper substance protects it in the stomach and lets a small share through.
The semaglutide tablet works in the stomach, right next to the tablet.
  1. The tablet dissolves in the stomach. It contains semaglutide and a helper substance, SNAC (salcaprozate sodium).
  2. A local shield. Around the dissolving tablet, SNAC buffers the acid, which protects semaglutide from the stomach’s enzymes.
  3. A short-lived door. SNAC briefly helps semaglutide pass through the stomach-lining cells themselves, not between them.
  4. A small share gets in. About 1–2% of the dose reaches the blood. It varies a lot from person to person.
  5. Food and water get in the way. Absorption falls if the tablet is taken with food or a lot of water.

Sources 12, 6

This works because semaglutide is potent and has a half-life of about a week, so a small, variable daily share adds up to steady levels.6,7,8 The cost: so little gets in that the tablet has to be taken every day under strict rules, and how much reaches the blood varies a lot between people.6

In detail What the tablet costs: rules, variation and switching

The evidence for the mechanism

In the developer’s studies in dogs and people, semaglutide from the tablet was absorbed in the stomach, close to the tablet surface, and only when it was formulated with SNAC . SNAC protected against enzyme breakdown by buffering the acid nearby and only briefly increased absorption. Absorption went through the cells, with no sign of an effect on the tight junctions between them.12 The EU label adds that, based on laboratory data, SNAC helps absorption, and that absorption happens mainly in the stomach.6

The rules on the label

Because food and water reduce absorption, the EU label for Rybelsus asks people to take the tablet on an empty stomach after fasting for at least eight hours, with no more than half a glass of water, and to wait at least 30 minutes before eating, drinking or taking other tablets.6 Your prescriber explains how this fits your day.

Variation between people

The share absorbed varies widely: the label gives a between-person variation of about 100%, and states that 2 to 4 in 100 patients will not have any exposure at all.6 For the same reason, the effect of switching between the tablet and the injection cannot easily be predicted.6 In clinical trials the tablet still lowered blood sugar: in PIONEER 1, a randomised, double-blind, placebo-controlled trial in 703 adults with type 2 diabetes, with authors from the manufacturer, it lowered HbA1c (average blood sugar) compared with placebo at all three tablet strengths over 26 weeks.13

The other route: a small-molecule pill

Is there a pill that acts like a peptide?

Yes. Orforglipron is not a peptide, but it switches on the same GLP-1 receptor.
A pill that is not a peptide gets around the problem the peptide tablet has. Semaglutide tablets and orforglipron, from their labels and regulators. Status checked on 22 October 2026.
Semaglutide tablet (a peptide)Orforglipron tablet (not a peptide)
Gets into the blood well when swallowed?No About 1–2% of the dose6Yes About 77% of the dose, says the US label14
Must be taken on an empty stomach?Yes Yes, with strict rules6No With or without food14,15
Approved in the US?Yes Yes16Yes Since 1 April 2026, for weight management17
Authorised in the EU?Yes Since 2020 (Rybelsus)6No Not yet. The EMA’s medicines committee recommended it on 15 October 2026; the European Commission had not yet decided18,15

Instead of protecting a large peptide, chemists found a small molecule that switches on the same receptor. At 883 daltons it is roughly a fifth the size of semaglutide.19,20 It is a partial agonist with its own profile, not a peptide in pill form (see how signalling works).21

In detail Orforglipron: what is known, and what is different
  • What it is. In laboratory and structural studies by its developers, orforglipron (code LY3502970) bound its own pocket in the GLP-1 receptor, touching different parts of the receptor than GLP-1 does, acted as a partial agonist, and favoured the G-protein signal .21,19
  • Evidence in people. In a manufacturer-funded, randomised, double-blind phase 2 trial in 272 adults with obesity, or with overweight and a weight-related condition, and without diabetes, mean weight change at 36 weeks ranged from −9.4% to −14.7% across dose groups, against −2.3% with placebo; stomach and gut side effects led 10 to 17 in 100 to stop .22 The EMA’s recommendation rests on two weight-management studies in 4,740 adults and four diabetes studies in 3,765 adults.15
  • Different trade-offs. Orforglipron is broken down mainly by the liver enzyme CYP3A4, so the label adjusts its use with medicines that block or speed up that enzyme. Its half-life is about 29 to 49 hours.14 It does not act on the rat or mouse receptor, so the usual rodent tests could not show receptor-driven thyroid tumours; the US label still carries a boxed warning about thyroid tumours.14

Capsules, sprays and drops sold online

Do peptide capsules, sprays or drops work as well as injections?

There is no evidence that they do. No study in people has measured how much reaches the blood.
For the tablet, we know how much gets in. For BPC-157 capsules and sprays, we do not. Semaglutide tablets compared with BPC-157 sold as capsules, oral spray or nasal spray, from the EU label and the FDA's 2026 review.
Semaglutide tablet (authorised)BPC-157 capsules or sprays
Absorption measured in people?Yes About 1–2% reaches the blood6No No human data by mouth, nose or skin23
Built to get through the gut wall?Yes With SNAC, tested in dogs and people12Unknown No published evidence23
Trial results in people published for this form?Yes Large controlled trials13No A phase 1 study of BPC-157 tablets was registered in 2015; no results were posted24,23

The FDA found BPC-157 sold as capsules, oral sprays and nasal sprays, but no human data on how much of it reaches the blood by these routes.23 Claims that such products work as well as an injection have no published support. If you already have one, the product risk page explains what to check.

In detail What the evidence does and does not show

Taken by mouth

Some rat studies have reported effects of BPC-157 given in drinking water , and BPC-157 resisted gastric juice in a lab test.23 Neither shows that it reaches the blood in people. A rat study measures an effect, not absorption, and surviving stomach juice is only the first of the five barriers above. In two human studies of BPC-157 given as an enema, the authors reported that it was not detected in the blood.23

Under the tongue and in the nose

Taking a peptide under the tongue or in the nose avoids the stomach, but the lining is still a barrier. Approved nasal peptides exist. The US label for desmopressin tablets puts their bioavailability at about 0.16% of an injection into a vein and about 5% of the nasal spray; together, these figures mean the nasal spray delivers only about 3% of what a vein injection does.9 Semax is registered in Russia as nasal drops.25 For TB-500 and KPV, the FDA found no human studies of absorption by any route.26,27

What this means

Without measurements, nobody can say whether a capsule or spray delivers nothing, a little, or an unpredictable amount, or whether that changes from one batch to the next. A product that delivers little is not automatically harmless either: what is in it, including impurities, is still unknown (see product risk).

Sources

Every claim on this page links to one of these sources. Scholarly sources link to their DOI; regulatory sources to the regulator's own page.

In detail All 27 sources, numbered as in the text
  1. Zhu Q, Chen Z, Paul PK, Lu Y, Wu W, Qi J (2021). Oral delivery of proteins and peptides: Challenges, status quo and future perspectives. Acta Pharmaceutica Sinica B 11:2416-2448. Review doi:10.1016/j.apsb.2021.04.001 ↩
  2. Santiago M, Davis EA, Hinton T, Angelo TA, Shield A, Babey AM, et al. (2021). Defining and unpacking the core concepts of pharmacology education. Pharmacology Research & Perspectives 9:e00894. Research article doi:10.1002/prp2.894 ↩
  3. Drucker DJ (2020). Advances in oral peptide therapeutics. Nature Reviews Drug Discovery 19:277-289. Review doi:10.1038/s41573-019-0053-0 ↩
  4. Diao L, Meibohm B (2013). Pharmacokinetics and pharmacokinetic-pharmacodynamic correlations of therapeutic peptides. Clinical Pharmacokinetics 52:855-868. Review doi:10.1007/s40262-013-0079-0 ↩
  5. Lipinski CA, Lombardo F, Dominy BW, Feeney PJ (2001). Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Advanced Drug Delivery Reviews 46:3-26. Review doi:10.1016/s0169-409x(00)00129-0 ↩
  6. European Medicines Agency (2026). Rybelsus (semaglutide): EPAR — overview and product information. Official product information ema.europa.eu — accessed 20 October 2026 ↩
  7. European Medicines Agency (2026). Wegovy (semaglutide): EPAR — overview and product information. Official product information ema.europa.eu — accessed 20 October 2026 ↩
  8. Brayden DJ, Hill TA, Fairlie DP, Maher S, Mrsny RJ (2020). Systemic delivery of peptides by the oral route: Formulation and medicinal chemistry approaches. Advanced Drug Delivery Reviews 157:2-36. Review doi:10.1016/j.addr.2020.05.007 ↩
  9. U.S. Food and Drug Administration (Drugs@FDA) (2019). Desmopressin Acetate Tablets — prescribing information (revised 10/2019). Official product information accessdata.fda.gov — accessed 22 October 2026 ↩
  10. U.S. Food and Drug Administration (2026). Drugs@FDA: NDA 050574 (Sandimmune cyclosporine oral solution). Regulator accessdata.fda.gov — accessed 22 October 2026 ↩
  11. Le Daré B, Ferron PJ, Gicquel T (2021). Toxic Effects of Amanitins: Repurposing Toxicities toward New Therapeutics. Toxins 13:417. Review doi:10.3390/toxins13060417 ↩
  12. Buckley ST, Bækdal TA, Vegge A, Maarbjerg SJ, Pyke C, Ahnfelt-Rønne J, et al. (2018). Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist. Science Translational Medicine 10:eaar7047. Research article doi:10.1126/scitranslmed.aar7047 ↩
  13. Aroda VR, Rosenstock J, Terauchi Y, Altuntas Y, Lalic NM, Morales Villegas EC, et al. (2019). PIONEER 1: randomized clinical trial of the efficacy and safety of oral semaglutide monotherapy in comparison with placebo in patients with type 2 diabetes. Diabetes Care 42:1724-1732. Clinical trial doi:10.2337/dc19-0749 ↩
  14. U.S. Food and Drug Administration / Eli Lilly and Company (2026). FOUNDAYO (orforglipron) tablets, for oral use — prescribing information (revised 07/2026). Official product information accessdata.fda.gov — accessed 22 October 2026 ↩
  15. European Medicines Agency (2026). New oral GLP-1 medicine for weight management and type 2 diabetes. Regulator ema.europa.eu — accessed 22 October 2026 ↩
  16. U.S. Food and Drug Administration (2026). Drugs@FDA: NDA 213051 (Rybelsus and Ozempic semaglutide tablets). Regulator accessdata.fda.gov — accessed 20 October 2026 ↩
  17. U.S. Food and Drug Administration (2026). FDA Approves First New Molecular Entity Under National Priority Voucher Program. Regulator fda.gov — accessed 22 October 2026 ↩
  18. European Medicines Agency (2026). Foundayo (orforglipron): opinion page (EMEA/H/C/006632). Regulator ema.europa.eu — accessed 22 October 2026 ↩
  19. National Center for Biotechnology Information (2026). PubChem Compound Summary for CID 137319706, Orforglipron. PubChem. Dataset pubchem.ncbi.nlm.nih.gov — accessed 22 October 2026 ↩
  20. National Center for Biotechnology Information (2026). PubChem Compound Summary for CID 56843331, Semaglutide. PubChem. Dataset pubchem.ncbi.nlm.nih.gov — accessed 20 October 2026 ↩
  21. Kawai T, Sun B, Yoshino H, Feng D, Suzuki Y, Fukazawa M, et al. (2020). Structural basis for GLP-1 receptor activation by LY3502970, an orally active nonpeptide agonist. Proceedings of the National Academy of Sciences of the United States of America 117:29959-29967. Research article doi:10.1073/pnas.2014879117 ↩
  22. Wharton S, Blevins T, Connery L, Rosenstock J, Raha S, Liu R, et al. (2023). Daily Oral GLP-1 Receptor Agonist Orforglipron for Adults with Obesity. The New England Journal of Medicine 389:877-888. Clinical trial doi:10.1056/NEJMoa2302392 ↩
  23. U.S. Food and Drug Administration (2026). FDA Briefing Document for BPC-157-Related Bulk Drug Substances (BPC-157 (free base) and BPC-157 acetate), Pharmacy Compounding Advisory Committee meeting, 23–24 July 2026. Regulator fda.gov — accessed 20 October 2026 ↩
  24. ClinicalTrials.gov (2015). NCT02637284 — Phase I, pilot study in healthy volunteers, to assess the safety and pharmacokinetics of PCO-02, which active ingredient is BPC-157. Trial registration clinicaltrials.gov — accessed 20 October 2026 ↩
  25. U.S. Food and Drug Administration (2026). FDA Briefing Document for Semax-Related Bulk Drug Substances (Semax (free base) and Semax acetate), Pharmacy Compounding Advisory Committee meeting, 23–24 July 2026. Regulator fda.gov — accessed 22 October 2026 ↩
  26. U.S. Food and Drug Administration (2026). FDA Briefing Document for TB-500-Related Bulk Drug Substances (TB-500 (free base) and TB-500 acetate), Pharmacy Compounding Advisory Committee meeting, 23–24 July 2026. Regulator fda.gov — accessed 20 October 2026 ↩
  27. U.S. Food and Drug Administration (2026). FDA Briefing Document for KPV-Related Bulk Drug Substances (KPV (free base) and KPV acetate), Pharmacy Compounding Advisory Committee meeting, 23-24 July 2026. Regulator fda.gov — accessed 20 October 2026 ↩