Peptide basics

Semaglutide weighs 4,114. Aspirin weighs 180. That is why one is a needle

This page explains why route of administration differs between peptides and ordinary drugs. It describes chemistry, not practice: it contains no dose, no schedule, no route instructions and no guidance on administering anything.

Two problems, not one

The first problem is chemical. A peptide is a chain of amino acids joined by peptide bonds, and the digestive system exists in large part to cut exactly those bonds. Swallowing a peptide presents the stomach and small intestine with something they are specifically equipped to dismantle.⁴

The second is size. Even a molecule that survived digestion has to cross the intestinal wall, and that gets rapidly harder as molecules get bigger. Conventional oral drugs cluster below a molecular weight of about 500 for this reason.¹²

The numbers on this site

• Tesamorelin: 5,136 g/mol.
• Tirzepatide: 4,813 g/mol.
• Semaglutide: 4,114 g/mol.
• BPC-157: 1,419.5 g/mol.
• Oxytocin: 1,007.2 g/mol.
• Glutathione: 307.33 g/mol.¹²⁴

The spread is wide because peptide is a structural category rather than a size class. Glutathione is a tripeptide and behaves in many respects like a small molecule. Tesamorelin is forty-four amino acids long and does not.²⁴

The exception that proves the mechanism

RYBELSUS is an approved oral tablet containing semaglutide — the same 4,114-weight molecule that is otherwise injected. Its existence looks like a refutation and is actually a confirmation.³

The tablet does not shrink the molecule. It pairs it with an absorption enhancer that briefly makes the stomach lining more permeable, and accepts that only a small fraction of the swallowed amount will be absorbed. The problem was solved by engineering around it, at considerable cost, for one molecule.³

What this means for oral products sold online

Capsules, sprays and drops containing peptides are widely sold. The chemistry above does not say they contain nothing; it says that getting an intact peptide from the gut into the bloodstream is difficult enough that an approved product required a purpose-built formulation and a regulatory review.³

The question worth asking of any oral peptide product is what specifically it does about absorption, and whether that has been measured in people. For approved products the answer is in the label. For most others there is no answer.³

Common questions