Peptides are short chains of amino acids, the same small molecules that link together to build every protein in the body. What separates a peptide from a protein is mostly length, and in the United States the dividing line is set by regulation rather than by chemistry. FDA defines a protein as a chain of more than 40 amino acids [1].
Many of the body’s own signals are peptides, and many drugs copy them. The best-known peptide drugs today are the GLP-1 medicines, and the evidence for those is in tirzepatide vs semaglutide. Most of the other peptides readers ask about have far less behind them, and the value of knowing what a peptide is lies in knowing which questions to ask about each one.
How a chain of amino acids becomes a signal
Each amino acid is joined to the next by a peptide bond, and the order of amino acids decides the shape the chain folds into. That shape is what lets a peptide fit a receptor on the surface of a cell, much as a key fits one lock, and once it binds, the cell switches a process on or off.
The receptor, not the word “peptide,” decides what happens next. Sermorelin, a 29-amino-acid fragment of growth-hormone-releasing hormone, acts on the pituitary to release growth hormone [2]. Ipamorelin, only five amino acids long, reaches a similar result through a different receptor [3]. Bremelanotide, a ring-shaped chain of seven, acts on melanocortin receptors, which is why its label warns about skin darkening [4].
Where the 40-amino-acid line comes from
In 2020, FDA finalized a rule saying that any amino acid chain with a defined sequence longer than 40 amino acids is a “protein” and is regulated as a biological product [1]. Chains at or under that length stay on the drug side, which is where semaglutide, tirzepatide, bremelanotide and sermorelin all sit.
The line has consequences you might not expect. Insulin, which a 2022 review describes as a peptide of 51 amino acids [5], falls above it. One public comment on the rule supported it for exactly that reason: it brings insulin into the pathway for biological products [1]. GLP-1 itself is 37 amino acids long, just under the line.
Why most peptide drugs are injected
The same bonds that hold a peptide together are what the gut is built to break. A review of therapeutic peptides describes how amide bonds are readily hydrolyzed by enzymes, leaving natural peptides unstable with a short half-life and fast elimination [5].
So most peptide drugs are injected under the skin, and much of the chemistry behind the newer ones goes into making them last longer once they are there. Swapping in unnatural amino acids at the points enzymes attack, for example, is one strategy that review describes for extending half-life. The same review notes that natural GLP-1 has a very short half-life, and modified versions of it became the top-selling peptide drugs for type 2 diabetes. Sermorelin, an older peptide, was given as a daily injection in its trials.
How many peptide drugs exist
Since insulin was introduced almost a century ago, more than 80 peptide drugs have reached the market, for conditions from diabetes and cancer to osteoporosis, HIV and chronic pain [6]. A separate review counts 33 non-insulin peptide drugs approved worldwide since 2000 alone [5].
Approved, compounded or research-only
For any peptide you meet, three questions do most of the work. Is it an approved drug with a label? Can a pharmacy legally compound it? Or is it sold only as a research chemical, with no approved use? The answers, peptide by peptide, are in are peptides legal.
After that comes the evidence itself, which varies enormously: large randomized trials for the weight-loss peptides, and a handful of small studies for many of the others. The goal pages, starting with longevity, sort the compounds by what readers want them for.