NAD+, methylene blue, and a rotating cast of others show up on peptide sites for one reason: that's where the buyers are.
the mismatch
A peptide is a short chain of amino acids joined by peptide bonds — that is the entire definition. A significant share of what sits in peptide vendor catalogues does not meet it. The category name has drifted from a chemical description into a market segment, and the two no longer overlap cleanly.
the usual suspects
NAD+ is a dinucleotide coenzyme, built from nucleotides rather than amino acids, and is not a peptide by any reading. Methylene blue is a synthetic dye and small molecule with a genuine pharmacological history that has nothing to do with this category. Glutathione is the interesting edge case — it genuinely is a tripeptide, so it belongs chemically even though it reaches the shelf via the supplement world rather than the research one.
NAD+, methylene blue, and a rotating cast of others show up on peptide sites for one reason: that's where the buyers are.
why it matters
Different molecular classes have different pharmacology, different failure modes, and completely separate evidence bases. Reasoning about NAD+ using intuitions built from peptide research produces nonsense in both directions. The shared shelf implies a kinship that the chemistry does not support.
the useful habit
When something appears in a peptide catalogue, check what it actually is before you read anything else about it. The category label is a merchandising decision, and merchandising decisions are not a guide to what a molecule does in a body.