Bioavailability

Bioavailability (denoted as F) represents the rate and extent to which an active drug ingredient is absorbed from its pharmaceutical form and becomes available at the site of physiological action. Intravenous administration provides 100% bioavailability (F = 1.0), whereas subcutaneous, oral, or intranasal routes exhibit lower bioavailability due to incomplete absorption, local enzymatic degradation, or first-pass hepatic metabolism.

Because peptides are vulnerable to gastric acid and gastrointestinal proteases, oral peptide bioavailability is typically less than 1%. Oral semaglutide (Rybelsus) overcomes this using the absorption enhancer SNAC, achieving ~0.4–1% bioavailability.

Bioavailability is the reason most peptide drugs are injected. The digestive system is built to break proteins and peptides into single amino acids, and the gut wall is a strong barrier to large molecules. As a result, only a small fraction of a peptide swallowed as a tablet usually reaches the bloodstream intact. When a product claims to deliver a peptide by mouth, under the tongue or through the skin, bioavailability is the first question to ask.

Oral semaglutide (Rybelsus) shows both what is possible and how hard it is. It works only because it is co-formulated with an absorption enhancer, SNAC, which research found helps it cross the stomach lining and protects it locally from enzymes. Even so, its oral bioavailability is around 1% or less, which is why the tablet comes with strict instructions on how it must be taken.

For research-chemical peptides sold as capsules, nasal sprays or creams, there is usually no published bioavailability data at all. Without it, there is no basis for assuming that a meaningful amount of the peptide reaches the body, let alone its target. A route that sounds convenient is only useful if studies show the peptide actually gets through, and for most of these products nobody has checked.

“If it works as an injection, it works as a pill”

Route of delivery changes everything for peptides. Stomach acid, digestive enzymes and the intestinal wall destroy or block most of an oral dose. Oral semaglutide needed a dedicated absorption enhancer and years of development to become viable. An oral or sublingual version of another peptide cannot be assumed to work without its own absorption data.

“High bioavailability means a stronger drug”

Bioavailability describes how much reaches the bloodstream, not how potent the drug is once there. A drug with low bioavailability can still be effective if it is very potent or given in a formulation that compensates, as oral semaglutide shows. Effect depends on bioavailability, potency, half-life and the dose together.

“Bioavailability is a fixed number”

Published values are averages from specific studies and formulations. Real-world absorption can vary between people and with factors such as food, timing, injection site and formulation. That is why approved labels give precise instructions on how a product should be taken, and why figures should not be carried across from one product to another.

Why can’t peptides be taken orally?

Most peptides are digested like dietary protein: stomach acid and gut enzymes cut them into fragments, and the intestinal lining lets very little of an intact peptide through. Some can be taken orally with special technology, such as oral semaglutide with the absorption enhancer SNAC, but bioavailability remains very low. Most peptide drugs are therefore given by injection.

What is the bioavailability of oral semaglutide?

Oral semaglutide (Rybelsus) has an absolute bioavailability of roughly 0.4% to 1%, compared with about 89% for the subcutaneous injection. It is absorbed mainly in the stomach, close to the tablet, with the help of SNAC. That small fraction is enough because semaglutide is potent and stays in the body for about a week.

Do peptide nasal sprays or creams work?

Some approved peptide drugs are given nasally, but each relies on formulation work and studies showing that enough reaches the bloodstream. For most research-chemical peptides sold as sprays, sublingual drops or creams, no published human absorption data exist, so there is no evidence on how much, if any, is absorbed.