Bioavailability: Why Route of Administration Decides Everything
A compound that does nothing orally can be active by injection. Bioavailability is the number that explains why.
Last reviewed September 10, 2026
Bioavailability is the proportion of an administered amount that reaches the bloodstream unchanged. Intravenous administration is defined as 100 per cent; every other route is measured against it.
Why peptides struggle orally
Peptides are chains of amino acids, and the digestive system exists to break those chains apart. Stomach acid and intestinal enzymes degrade most peptides before absorption, and what survives is often too large to cross the gut wall efficiently.
Oral peptide products that do work in medicine achieve it with absorption enhancers, protective formulation or chemical modification — deliberate engineering, not a happy accident.
What this means when reading claims
If a study demonstrated an effect by injection, that finding does not transfer to a capsule, a spray or a cream of the same compound. Route is part of the evidence, not a delivery detail.
Topical and intranasal routes each have their own absorption literature, and it is compound-specific. Assume nothing; look for the pharmacokinetic study.