Bioavailability

This is the reason almost everything here is injected. Peptides are chains of amino acids and the digestive system exists to break those chains, so an oral peptide is mostly destroyed before it is absorbed.

Where an oral version exists, it usually means one of two things. Either the compound is not a peptide, as with MK-677 and 5-Amino-1MQ, or it has been engineered with an absorption enhancer. This is what oral semaglutide does.

Nasal and topical routes have their own limits, and a compound with human evidence by one route does not carry that evidence to another. GHK-Cu's human data is topical; injecting it is a different question with a different, thinner answer.