Why most peptides are injected
The gut is built to digest proteins and peptides into their component amino acids. An oral peptide therefore faces low oral bioavailability: stomach acid and digestive enzymes degrade it, and little or none reaches the bloodstream intact. Injecting a peptide bypasses digestion entirely, which is why the great majority of therapeutic and research peptides are given as injections — most commonly subcutaneous (into the fat layer just under the skin).
Oral peptides: the exceptions
A few peptides do work by mouth, through clever workarounds:
- Special formulations. Oral semaglutide (Rybelsus) is paired with an absorption enhancer that helps it survive the stomach.
- Gut-local action. Peptides like linaclotide and plecanatide are meant to act in the gut itself, so they don't need to be absorbed into the bloodstream.
Nasal sprays
The nasal lining offers another route that avoids the digestive tract. Several neuropeptides studied for the brain — such as Semax and Selank — are formulated as intranasal solutions, partly because this can offer more direct access toward the central nervous system.
Topical (skin) peptides
Cosmetic peptides are applied to the skin as ingredients in serums and creams. Copper peptide GHK-Cu, Matrixyl, and Argireline are designed to act locally in the skin rather than be absorbed systemically, and are regulated as cosmetics.
Frequently asked questions
Why can't most peptides be taken orally?
Peptides are short amino-acid chains, and the digestive system breaks them down before they can be absorbed. This gives most oral peptides very low bioavailability, which is why the majority are injected (usually subcutaneously) to bypass digestion.
Are there any oral peptides?
Yes, a few. Oral semaglutide (Rybelsus) uses an absorption enhancer to survive the stomach, and gut-targeted peptides like linaclotide and plecanatide are designed to act in the intestine itself, so they don't need to enter the bloodstream.
Why are some peptides given as nasal sprays?
The nasal route avoids the digestive tract and can offer more direct access toward the brain, which is why several neuropeptides such as Semax and Selank are formulated as intranasal solutions.
How do peptide skincare products work?
Cosmetic peptides like GHK-Cu, Matrixyl, and Argireline are applied topically and are designed to act locally within the skin rather than be absorbed into the body. They are regulated as cosmetics.
Further reading
Selected peer-reviewed sources on this topic, labelled by type. A citation is a reference, not an endorsement.
- Zhang X, Han Y, Huang W, et al. The influence of the gut microbiota on the bioavailability of oral drugs. Acta Pharm Sin B. 2021. Peer-reviewed study
- Agrawal M, Saraf S, Saraf S, et al. Nose-to-brain drug delivery: An update on clinical challenges and progress towards approval of anti-Alzheimer drugs. J Control Release. 2018. Peer-reviewed study
- Parida P, Prusty AK, Patro SK, et al. Current Advancements on Oral Protein and Peptide Drug Delivery Approaches to Bioavailability: Extensive Review on Patents. Recent Adv Drug Deliv Formul. 2024. Peer-reviewed study