Oral vs Injectable Peptides: What Actually Gets Absorbed

Nearly every peptide drug is injected, and people reasonably ask why. If one GLP-1 drug now comes as a tablet, why not all of them? And what about capsules, nasal sprays and creams sold for other peptides? The short answer is that the body is very good at keeping peptides out. This guide explains the barriers, the one well-studied way around them, and where the evidence for other routes is thin.

Why most peptides are not orally bioavailable

Bioavailability is the share of a dose that reaches the bloodstream intact. For injected drugs it is high by design. For swallowed peptides it is usually close to zero, for two reasons (Buckley 2018):

1. Digestion. The stomach and intestine are full of proteolytic enzymes whose job is to cut proteins and peptides into single amino acids. A therapeutic peptide looks like food to them.
2. The epithelial barrier. Even a peptide that survives digestion has to cross the gut lining. Peptides are large and water-loving compared with typical tablet drugs, which makes that crossing difficult.

A 2026 review of the field concluded that most oral peptide programs have failed despite a century of effort. It identified three recurring failure modes: exposure that is too low when poor absorption combines with a short half-life; absorption so variable from dose to dose that regulators cannot accept it; and doses that have to rise so high that gut side effects and manufacturing costs become prohibitive (Niazi 2026).

That is why subcutaneous injection, just under the skin, remains the default route for peptides.

How oral semaglutide works

Oral semaglutide is the exception that shows how hard the problem is. The tablet combines the peptide with an absorption enhancer called SNAC (sodium N-[8-(2-hydroxybenzoyl) aminocaprylate]). Studies in people and dogs found that (Buckley 2018):

• Absorption happens in the stomach, not the intestine, in a small area close to the dissolving tablet.
• It requires SNAC. SNAC protects the peptide from enzymes through a local buffering effect and only transiently enhances absorption.
• The peptide crosses through the lining cells themselves, with no evidence of loosening the tight junctions between cells.

Even with this engineering, very little gets through. In 22 healthy volunteers, the bioavailability of a 3 mg oral dose relative to a 0.25 mg subcutaneous dose was 0.66% (Bouhajib 2025).

So why does it work at all? The same review argues that oral semaglutide is a boundary case, not a template. It succeeds because of a rare combination: an exceptionally long half-life (about 168 hours), high potency, a wide therapeutic window, and effects that depend on exposure built up over time, which together tolerate low and variable absorption (Niazi 2026). Oral semaglutide has been studied in randomized controlled trials in the PIONEER program in type 2 diabetes (Clements 2021). Most peptides have half-lives of minutes to hours and would never accumulate from such tiny absorbed amounts.

The review also notes that small-molecule GLP-1 agonists such as orforglipron take a different route to oral dosing altogether: they are not peptides, so the peptide absorption problem does not apply to them in the same way (Niazi 2026).

What is actually known about oral BPC-157

Oral BPC-157 is widely marketed, so it is worth being precise about the evidence.

• The stability claim. A 2011 review of BPC-157's gastrointestinal research describes it as stable in human gastric juice (Sikiric 2011).
• The animal data. In rats with a cut knee ligament, BPC-157 given in drinking water improved healing, as did injected and topical forms (Cerovecki 2010). Results like this are the basis for oral use.
• The pharmacokinetic data. The first formal pharmacokinetic study, in rats and dogs, tested intravenous and intramuscular injection. It found a half-life under 30 minutes and intramuscular bioavailability of about 14–19% in rats and 45–51% in dogs (He 2022). Oral absorption was not part of that study.
• The human data. A 2026 review described BPC-157's human pharmacokinetics as critically undercharacterized, noted that no pharmaceutical-grade formulation has been developed, and counted fewer than 30 human subjects across three uncontrolled pilot studies (Mateescu 2026).

The honest summary: oral activity has been reported in rats. How much oral BPC-157 reaches the bloodstream in people has not been published in the studies we reviewed. Claims about oral BPC-157 in humans are extrapolations from animal work.

The intranasal route

The nose offers a thin, blood-rich lining and skips the digestive enzymes of the gut, which is why some peptides are studied as nasal sprays (intranasal). The evidence shows both the promise and the limits.

• GHRH(1-29) in people. In 30 healthy men, nasal absorption was low, with bioavailability of only 3–5%. A nasal dose of about 50 µg/kg was roughly as potent as 1 µg/kg injected intravenously (Wilton 1993).
• Semax in rats. Using radioactively labeled Semax, researchers found 0.093% of the administered radioactivity per gram of brain tissue two minutes after a nasal dose. The peptide was also rapidly broken down by enzymes (Shevchenko 2006).
• Selank in mice. Nasal and injected Selank produced different patterns of brain receptor changes, which the authors attributed to differences in how each route is absorbed and metabolized (Vasil'eva 2016).

Most of the Semax and Selank delivery research is in animals and was published in Russian-language journals, with limited independent replication. The Semax vs Selank comparison summarizes their broader evidence.

Topical peptides and the skin barrier

Skin is a barrier by design. A widely cited review proposed a "500 Dalton rule": a compound's molecular weight generally needs to be under 500 Daltons to pass the outer layer of skin, the stratum corneum (Bos 2000). Most peptides are much larger. BPC-157, for instance, has a molecular weight of 1419 (Sikiric 2011).

Measurements bear this out. When a cream containing 10% acetyl hexapeptide-8 (the Ac-EEMQRR sequence sold as Argireline) was applied to human skin in the lab, most of it washed off the surface. About 0.22% of the dose was found in the stratum corneum, 0.01% in the epidermis, and none was detected in the dermis (Kraeling 2015). A review of anti-wrinkle peptides concluded that most are not good candidates for skin permeation without enhancement methods such as microneedles, iontophoresis or nanocarrier encapsulation (Mortazavi 2022).

Copper peptides are a partial exception. In a lab study on human skin, copper applied as GHK-Cu permeated the skin and was also retained in it (Hostynek 2010). Note that the study measured copper, not the intact peptide.

Summary

The route matters as much as the molecule. A result obtained with injection does not automatically apply to a capsule, a spray or a cream.

Sources

• Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist. Sci Transl Med, 2018. PMID 30429357
• Oral delivery of peptides and proteins: pharmacokinetic boundaries, negative selection, and route triage. Front Drug Deliv, 2026. PMID 41939723
• The pharmacokinetics and comparative bioavailabilty of oral and subcutaneous semaglutide in healthy volunteers. J Basic Clin Physiol Pharmacol, 2025. PMID 40425315
• Pharmacokinetics and Clinical Implications of Oral Semaglutide for Type 2 Diabetes Mellitus. Clin Pharmacokinet, 2021. PMID 33108617
• Stable gastric pentadecapeptide BPC 157: novel therapy in gastrointestinal tract. Curr Pharm Des, 2011. PMID 21548867
• Pentadecapeptide BPC 157 (PL 14736) improves ligament healing in the rat. J Orthop Res, 2010. PMID 20225319
• Pharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol, 2022. PMID 36588717
• BPC-157 as an Investigational Peptide Therapeutic: Biopharmaceutical Challenges, Formulation Strategies, and Translational Development Barriers. Pharmaceutics, 2026. PMID 42198317
• Pharmacokinetics of growth hormone-releasing hormone(1-29)-NH2 and stimulation of growth hormone secretion in healthy subjects after intravenous or intranasal administration. Acta Paediatr Suppl, 1993. PMID 8329825
• [Kinetics of Semax penetration into the brain and blood of rats after its intranasal administration]. Bioorg Khim, 2006. PMID 16523722
• [COMPARISON OF PHARMACOLOGICAL EFFECTS OF HEPTAPEPTIDE SELANK AFTER INTRANASAL AND INTRAPERITONEAL ADMINISTRATION TO BALB/c AND C57BL/6 MICE.]. Eksp Klin Farmakol, 2016. PMID 29787664
• The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Exp Dermatol, 2000. PMID 10839713
• In vitro skin penetration of acetyl hexapeptide-8 from a cosmetic formulation. Cutan Ocul Toxicol, 2015. PMID 24754410
• Skin permeability, a dismissed necessity for anti-wrinkle peptide performance. Int J Cosmet Sci, 2022. PMID 35302659
• Human skin retention and penetration of a copper tripeptide in vitro as function of skin layer towards anti-inflammatory therapy. Inflamm Res, 2010. PMID 20703511

This article is for educational and research purposes only and is not medical advice.