Bioavailability
Bioavailability is the fraction of an administered dose that reaches systemic circulation in active form. Subcutaneous injection of peptides typically achieves 80–100% bioavailability, while oral administration is usually 1–5% for most peptides.
Overview
Bioavailability is expressed as a percentage of the administered dose. Intravenous injection is defined as 100% bioavailability since the compound enters circulation directly. Subcutaneous injection is close behind for most peptides because fatty tissue is highly vascularized and peptides are rapidly absorbed. Oral bioavailability is dramatically lower for peptides due to enzymatic degradation in the gut and poor absorption across intestinal membranes. BPC-157 is a notable exception, showing measurable oral efficacy in animal studies, possibly due to its resistance to gastric acid. Intranasal delivery (used for Semax and Selank) offers intermediate bioavailability by bypassing first-pass metabolism while avoiding injection.
Related Terms
Frequently asked questions
Why is subcutaneous bioavailability so high for peptides?+
The subcutaneous layer contains a dense capillary network. Small peptide molecules diffuse quickly into blood vessels, and without passing through the digestive tract, they are not broken down by gut enzymes.
Does bioavailability affect how a dose is calculated?+
Yes. If a peptide has lower bioavailability via a particular route, a higher administered dose is needed to achieve the same systemic exposure as the same dose via a more bioavailable route.
Is oral bioavailability of peptides ever clinically meaningful?+
Rarely for most peptides, but BPC-157 animal studies suggest oral activity in the gut and systemically. GLP-1 receptor agonists like oral Semaglutide (Rybelsus) use absorption enhancers to achieve clinically meaningful oral bioavailability.