A peptide is a short chain of amino acids, usually 2 to 50, that works as a signal in the body.
Letters, words, paragraphs
Amino acids are letters. Peptides are short words. Proteins are entire paragraphs.
A peptide is a chain of amino acids linked by peptide bonds, typically 2 to 50 of them. Once a chain exceeds roughly 50 amino acids and folds into a complex three-dimensional shape, it stops being called a peptide and starts being called a protein. Insulin, at 51 amino acids, sits right on that boundary.
That size is not trivia. It determines almost everything practical about how these compounds behave: whether they survive your stomach, whether they cross into the brain, how fast they clear, and how specifically they bind.
Your body already runs on them
This is the part most marketing copy skips, and it is the part that makes the whole field make sense.
Your body naturally produces thousands of peptides, and they regulate nearly every biological process you have. The endorphins that reduce pain are peptides. Ghrelin, the hormone that signals hunger, is a peptide. The growth hormone-releasing hormones that trigger tissue growth are peptides. DSIP, the sleep compound covered later in this course, occurs naturally in your brain.
BPC-157 is derived from a protective protein found in human gastric juice. GHK-Cu is present in your blood plasma right now. MOTS-C is encoded in your own mitochondrial DNA.
Synthetic peptides mimic or amplify signals your body is already sending. That is a meaningfully different proposition from introducing a molecule biology has never seen, though it is not, on its own, a safety guarantee.
Specificity is the point
What makes peptides interesting as compounds is specificity.
Because they are small and bind particular receptors, peptides tend to target specific pathways rather than acting broadly across the body. A GLP-1 agonist acts on GLP-1 receptors. A GHRH analog acts on GHRH receptors on the anterior pituitary. That precision is why side effect profiles for well-characterised peptides are often narrower than for small-molecule drugs that hit many targets at once.
The scientific establishment has noticed. About 11% of all FDA-approved drugs between 2016 and 2024 were synthetic peptides, including insulin, semaglutide and tesamorelin.
Why almost everything is injected
Peptides are made of amino acids. Your digestive system is very good at breaking down amino acid chains. That is its job.
So most research peptides are administered by subcutaneous injection, because taking them orally means digesting them before they can do anything. The compound ships as a freeze-dried powder, is reconstituted with bacteriostatic water, and is injected with an insulin syringe into the abdomen, thigh or upper arm.
There are exceptions worth knowing. BPC-157 is stable in gastric acid, which is unusual and is why oral dosing appears in gut protocols. MK-677 is orally active. Some compounds have nasal spray formulations. GHK-Cu works topically because at around 340 daltons it is small enough to penetrate skin.
But injection remains the standard, and any protocol you read should specify the route rather than leaving you to guess.
What this course will not do
It will not tell you what to take. Every dosing figure you meet from here on is a commonly researched range: what appears in published literature and practitioner protocols, not a recommendation. That distinction holds for the entire course.