What does 'bioavailability' mean and why does it come up so often with peptides?
Updated: 2026-09-09 · Basics
Short answer
Bioavailability is the fraction of a substance that reaches the circulation intact. Intravenously it is 100% by definition; by other routes it is lower. For peptides it is usually very low by mouth, because they are small proteins and the digestive tract breaks them down — unless a formulation is specifically designed to prevent that.
In plain English
- It measures how much reaches the blood, not whether something works.
- The route of administration changes the result dramatically.
- A cream or spray is not automatically equivalent to the route used in a trial.
- Oral formulations with absorption enhancers exist; they are the exception, not the rule.
What we know
- Bioavailability is a standard pharmacology parameter and appears in regulatory labels.
- Enzymatic breakdown of peptides in the gut is well described.
What we don't know yet
- Most peptides sold as research material have no human bioavailability data.
- High bioavailability does not imply clinical benefit.