Bioavailability

Intravenous injection is defined as 100% bioavailability since the compound enters circulation directly. Subcutaneous and intramuscular injections achieve high bioavailability (often 70–100%) for most peptides. Oral bioavailability of peptides is typically near zero because stomach acid and digestive enzymes break peptide bonds before absorption.

This is why nearly all research peptides are injectable — the route of administration is chosen specifically to maximize the fraction of the dose that reaches the bloodstream intact. When comparing two administration routes, bioavailability differences must be factored into dose calculations.