Routes of Administration, Explained

Peptides are delicate molecules. How you get one into the body has a lot to do with whether it arrives in one piece and actually reaches your blood. This guide walks through the main ways peptides are given, and the one idea—bioavailability—that ties them all together. It’s for learning only: no doses, no protocols, no injection how-to.

What “bioavailability” means

Bioavailability is just the share of a dose that actually reaches your bloodstream in a working form. To keep things comparable, an intravenous (IV) dose—one put straight into a vein—is treated as 100%, since it goes right into the blood. Every other route is measured against that 100% mark, and it’s usually lower. The reason: the peptide first has to be absorbed, and for some routes it also has to pass through the liver, before it ever reaches the blood.

You’ll see it written as F (or as a percentage). Two things set it: how fast the peptide is absorbed, and how much of it gets absorbed. For peptides—which are chains of amino acids, the same building blocks our food is made of—the “how much” part is where most routes run into trouble. The body is built to digest them.

Subcutaneous injection: the default for most peptides

Most therapeutic and research peptides are injected subcutaneously—meaning into the layer of fat just under the skin—for one straightforward reason: it skips the digestive system entirely. That fatty layer has plenty of tiny blood and lymph vessels to carry the peptide away, but it doesn’t have the harsh digestive setting of the gut. So peptides come through fairly intact here, often somewhere in the rough range of about 65–100%, depending on the molecule.

It isn’t instant, though. The injection leaves a small pocket of peptide under the skin, and it seeps out slowly from there. That makes the blood-level curve slower, lower, and steadier than an IV dose. Some of the peptide gets broken down by enzymes right at the injection spot, and how much is absorbed can shift a bit depending on where you inject. So subcutaneous injection is dependable, but not perfectly even from one spot to the next.

Oral: why most peptides barely make it

Swallowing a peptide is easy and convenient, but for most of them it simply doesn’t work. The gut is a rough place. Stomach acid (very acidic, around pH 1–2) teams up with digestive enzymes—proteins called pepsin, trypsin, and chymotrypsin—to chop peptides into small pieces and loose amino acids. On top of that, the gut wall is bad at letting large, water-loving molecules through. Add it up and, for an unchanged peptide, the amount that reaches the blood by mouth is usually below 1%—sometimes far less.

A few products do beat the gut, and they show just how much engineering it takes. Oral semaglutide (sold as Rybelsus) is paired with an absorption helper called SNAC, and even then only about 1% gets through—which works only because the drug is unusually strong and long-lasting. Oral octreotide (Mycapssa, approved by the FDA in 2020) leans on a different trick, a “transient permeability enhancer” that briefly opens the gut wall. These are the rare exceptions, not a sign that peptides generally work when swallowed.

Intranasal and topical: niche routes

The lining inside the nose is thin, full of blood vessels, and skips that first trip through the liver—which makes it workable for a few small peptides. Desmopressin and salmon calcitonin nasal sprays have been used in the clinic for years. But the bigger the peptide, the worse the nose absorbs it. The nasal peptides on the market stay in the low-kilodalton range (a kilodalton, or kDa, is a unit for a molecule’s weight; salmon calcitonin, the largest of them, is about 3.4 kDa). For anything bigger, injection is still the standard.

Putting a peptide on the skin (topical) runs into an even tougher wall. The skin’s outer layer, the stratum corneum, stops most molecules heavier than about 500 daltons—a guideline often called the “500-dalton rule” (a dalton is the tiny unit used to weigh a single molecule). Common skincare peptides are heavier than that—for example, palmitoyl pentapeptide-4 at around 800 Da and acetyl hexapeptide-8 at around 890 Da. So without special delivery tech, those peptides mostly do their work on or near the skin’s surface rather than getting into the bloodstream.

Bottom line

The route isn’t a small detail—it decides whether a peptide does anything at all. Injection stays the default because peptides are made to be digested, and the gut, the skin, and even the nose are tough barriers to get past. Oral, nasal, and topical peptide products show up only when the molecule is small enough or strong enough, or when clever formulation gets it past the barrier. This is general education, not medical advice. Legal status, sourcing, and safety vary a lot, and any use should be talked through with a qualified clinician.

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