Why Route Matters
Bioavailability — the fraction of an administered compound that reaches systemic circulation in active form — varies dramatically for peptides depending on the administration route. A peptide showing strong effects via injection may be completely inactive orally. Understanding these differences is critical for research protocol design and for interpreting published literature that uses different routes.
Subcutaneous (SC) Injection
The most common route in peptide research. SC injection deposits the compound into the adipose layer beneath the skin, where it absorbs gradually into the bloodstream.
- Bioavailability: 50-80% (varies by peptide)
- Depot effect: Creates a slow-release reservoir, providing sustained levels
- Used for: Most research protocols — BPC-157, TB-500, semaglutide, tirzepatide
Intraperitoneal (IP) Injection
Widely used in rodent models. IP injection delivers compound into the peritoneal cavity for rapid absorption via the peritoneal membrane.
- Bioavailability: Higher peak levels than SC, faster clearance
- Used for: Most published preclinical BPC-157 studies, animal model research
- Limitation: Not practical for human research protocols
Oral Administration
Most peptides have near-zero oral bioavailability (<2%) due to gastric acid degradation and poor intestinal absorption. However, BPC-157 is a notable exception — multiple published studies report maintained biological activity via oral administration, attributed to its origin in gastric juice and remarkable acid stability.
Intranasal Delivery
The nasal mucosa provides direct access to the CNS via the olfactory nerve pathway, bypassing the blood-brain barrier. This makes intranasal delivery particularly relevant for nootropic peptides.
- Rapid CNS penetration: Minutes rather than hours
- Primary use: Selank and Semax were originally developed as intranasal preparations
- Limitation: Volume limited to ~100-200μL per nostril; enzymatic degradation by nasal enzymes
Factors Affecting Bioavailability
- Molecular weight: Smaller peptides (<1 kDa) generally absorb better than larger ones
- Lipophilicity: Increased lipophilicity improves membrane permeability (semaglutide's acyl chain exploits this)
- Enzymatic stability: Resistance to peptidases extends active exposure time
- Formulation: Solvent pH, excipients, and concentration affect absorption kinetics
For research purposes only.