Receptors and signals
A signaling peptide works by fitting a receptor — a protein on or in a cell built to recognize that specific shape — and flipping a switch. A GH-secretagogue peptide fits receptors that tell the pituitary to release growth hormone; a repair peptide fits receptors involved in cell migration and blood-vessel formation. The specificity is the point: a peptide that matches one receptor family tends not to rattle the rest of the system the way a blunt drug might. This is why peptides are described as working "upstream" — they pull a lever the body already uses, rather than overriding the machinery downstream.
Why most peptides are injected
Here's the practical consequence of "made of amino acids": your digestive system treats a swallowed peptide like food. Stomach acid and proteolytic enzymes break the chain apart, and what little survives struggles to cross the gut wall intact — for unmodified peptides, oral bioavailability is typically under 1%. So the standard route for research peptides is subcutaneous injection (into the fat just under the skin), which bypasses the gut entirely and delivers the intact molecule to circulation. A small insulin syringe is the usual tool, and the technique is simple enough to learn properly — covered in a later chapter.
There are exceptions that prove the rule: a few peptide drugs have been chemically modified or specially formulated to survive oral dosing. But for the typical research peptide, if it isn't injected, most of it never reaches the bloodstream.
Half-life: usually short, but not a universal law
How long a peptide lasts in the body — its half-life — shapes how often you dose it. Many research peptides clear quickly (minutes to a few hours), which is why some are dosed more than once a day or engineered with modifications (like the "DAC" on certain GH peptides) to extend their action. But "peptides have short half-lives" is an overgeneralization worth flagging: some peptide therapeutics last for days (the GLP-1 drugs are a notable example, with half-lives long enough for weekly dosing). The accurate version is that half-life varies enormously by peptide, and you dose each one according to its pharmacology, not a blanket rule.
Agonists, fragments, and analogs
You'll meet a few recurring terms. An agonist activates a receptor (most signaling peptides). A fragment is a piece of a larger natural peptide or protein — sometimes the active part, sometimes a different molecule than the full-length original (an important distinction that comes up with repair peptides). An analog is a modified version of a natural peptide, tweaked to last longer or hit a receptor harder. Knowing which one you're dealing with tells you a lot about how it behaves and how its evidence does or doesn't transfer from the natural molecule it's based on.
The honest limit of "it signals a pathway"
Because a peptide nudges a real pathway, it's tempting to assume the effect is large and guaranteed. It isn't necessarily either. Signaling a repair pathway doesn't mean dramatic healing in a human at a given dose; provoking a GH pulse doesn't guarantee a body-composition result. The mechanism being real is the floor for plausibility, not proof of a meaningful effect — and the gap between "plausible mechanism" and "demonstrated human benefit" is exactly where this course will keep you honest.
Educational content, not individual medical advice.