What is a peptide?
A short chain of amino acids. That sentence is true, and it is not a medical category.
The chemistry
A peptide is a chain of amino acids joined by amide bonds. Proteins are the same kind of chain, only longer, and often folded into a stable shape. There is no sacred length where a peptide becomes a protein. People often use “peptide” under about fifty residues and “protein” above that. Insulin sits near the border and is called both.
A small molecule is different chemistry: one compact structure, not a chain of amino acids. NAD+ is a dinucleotide coenzyme. It is on this site because people file it next to peptides. It is not one.
Sequence is the identity
Two vials can share a nickname and be different molecules. A single amino-acid swap, a protecting group, a salt, or a fatty-acid tail changes the mass and can change what the substance does. The sequence and the molecular weight on a certificate are the identity. The marketing name is not.
A blend is worse. BPC-157 mixed with TB-500 is two identities in one container. It does not get one formula, and a paper about one half is not a paper about the mixture.
Why the chain is interesting
Peptides can bind receptors and then be broken down by ordinary enzymes, which is why so many of them have short lives unless a chemist modifies them. Semaglutide is a GLP-1 analog with a side chain that lets it stick to albumin. That is a designed pharmaceutical, not a typical research-catalog fragment.
Binding a receptor is a mechanism. It is not, by itself, evidence that a person gets a useful outcome. The rest of this library is about that gap.
Sources for the named trials and approvals live on the compound pages and in the evidence explorer. This essay does not add new numbers.