A peptide is a short chain of amino acids—the same building blocks that make up proteins. Tirzepatide and semaglutide are peptides that mimic hormones your body already produces (GLP-1 and GIP). Unlike a small-molecule drug like ibuprofen, peptides are larger, fragile molecules that must be injected because stomach acid would destroy them. They're manufactured through chemical synthesis or recombinant DNA technology, which is why purity testing matters so much in the gray market.