Bioavailability
Also known as: Systemic availability, F value
Bioavailability is the proportion of a substance that enters systemic circulation and is available for biological activity when introduced into the body. For peptides, bioavailability varies significantly based on the route of administration, with injections typically providing near 100% bioavailability.
Last updated: February 1, 2026
Understanding Bioavailability
Bioavailability is expressed as a percentage:
- 100% bioavailability - The entire dose reaches systemic circulation (IV injection standard)
- 50% bioavailability - Half the dose reaches circulation
- 0% bioavailability - None of the substance reaches circulation
Bioavailability by Route of Administration
| Route | Typical Bioavailability | Notes |
|---|---|---|
| Intravenous (IV) | 100% (reference) | Direct bloodstream delivery |
| Intramuscular (IM) | 75-100% | Absorbed from muscle tissue |
| Subcutaneous (SC) | 70-100% | Absorbed from under skin |
| Oral | Less than 1% for most peptides | Destroyed by digestion |
| Nasal | 10-30% | Variable absorption |
| Transdermal | Very low | Poor peptide penetration |
Why Most Peptides Have Low Oral Bioavailability
Peptides face multiple barriers when taken orally:
- Gastric acid - Stomach acid denatures peptide structures
- Digestive enzymes - Proteases and peptidases break peptide bonds
- Poor membrane permeability - Peptides are too large and hydrophilic to cross intestinal walls
- First-pass metabolism - Liver further degrades absorbed peptides
Improving Peptide Bioavailability
Oral Delivery Technologies
- Absorption enhancers - SNAC (used in oral semaglutide) temporarily increases intestinal permeability
- Enteric coatings - Protect peptides from stomach acid
- Nanoparticle encapsulation - Shields peptides and aids absorption
Alternative Routes
- Sublingual - Absorption under the tongue avoids some digestive barriers
- Buccal - Absorption through cheek mucosa
- Pulmonary - Inhalation provides large surface area for absorption
Bioavailability in Practice
Oral Semaglutide (Rybelsus)
One of the few commercially successful oral peptides:
- Uses SNAC absorption enhancer
- Must be taken on empty stomach with minimal water
- Achieves ~1% bioavailability (enough for therapeutic effect due to high dose)
- Much lower than injectable (~89% bioavailability)
Subcutaneous Peptides
Most research peptides are administered subcutaneously because:
- High, consistent bioavailability
- Self-administration is possible
- Avoids digestive system entirely
Frequently Asked Questions
Why can’t I just take peptides as pills?
Most peptides are destroyed during digestion. They’re broken down by stomach acid and enzymes before they can be absorbed. The few oral peptide medications use special technologies to protect the peptide and enhance absorption.
Does higher bioavailability mean better results?
Higher bioavailability means more of the dose reaches your system, but optimal results depend on achieving the right concentration at the target site. Higher isn’t always better—it’s about reaching therapeutic levels.
How is bioavailability measured?
Bioavailability is calculated by comparing the area under the blood concentration-time curve (AUC) for a given route versus intravenous administration, which is defined as 100% bioavailable.
Related Peptides
Related Terms
Disclaimer: This glossary entry is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for medical questions.