Peptides: Bioavailability by Route

Bioavailability (BA) describes the fraction of an administered dose that reaches systemic circulation unchanged. For peptides, route of administration is the single most important factor — more so than dose or timing.

Why Oral Is Nearly Impossible

The gastrointestinal tract evolved to digest proteins and peptides. A peptide swallowed in a capsule faces three sequential barriers:

1. Gastric acid (pH 1.5–3.5) — denatures structure and initiates hydrolysis
2. Luminal peptidases (pepsin, trypsin, chymotrypsin, elastase) — cleave peptide bonds at specific residues
3. Brush-border peptidases (aminopeptidases, dipeptidyl peptidase IV) — final cleavage at the intestinal surface
4. Tight junctions — even intact fragments >500 Da cannot cross the epithelial barrier via paracellular transport

The combined effect: oral bioavailability of under 1% for most unformulated peptides [PMID 19879964].

Bioavailability by Route

Route	Bioavailability	Onset	Practical Notes	Examples
Intravenous (IV)	100%	Immediate	Rapid distribution; requires sterile technique; not practical for self-administration	Research settings
Subcutaneous (subcut)	85–95%	15–60 min	Forms depot; slow sustained release; practical for self-injection	BPC-157, ipamorelin, CJC-1295
Intramuscular (IM)	85–95%	10–30 min	Faster than subcut; depot still forms; slightly more discomfort	TB-500, some protocols
Intranasal	10–30%	5–15 min	Bypasses first-pass; useful for CNS-targeted peptides (PT-141, selank)	PT-141, selank, semax
Oral (unformulated)	<1%	Variable	Essentially not bioavailable; not recommended for most research peptides	N/A
Oral (SNAC formulation)	0.4–1%	30–60 min	Sufficient when peptide is extremely potent; complex pharmaceutical process	Semaglutide (Rybelsus)
Topical/transdermal	<5%	1–4 h	Only small, lipophilic peptides penetrate; GHK-Cu used cosmetically	GHK-Cu (cosmetic)

The SNAC Exception and What It Means

Semaglutide’s oral formulation (Rybelsus) is a pharmaceutical engineering achievement. SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate) works by: (1) locally raising gastric pH around the tablet, preventing acid degradation; and (2) acting as a permeation enhancer that transiently loosens the gastric mucosa. The result is 0.4–1% bioavailability — low in absolute terms, but sufficient because semaglutide binds the GLP-1 receptor with high affinity and has a 7-day half-life [PMID 31969683].

This formulation approach cannot be easily replicated in a home setting or by compounding pharmacies with standard equipment.

First-Pass Metabolism

Even peptides that survive GI degradation face hepatic first-pass extraction. Portal blood flows directly to the liver, where hepatic peptidases and proteases can further reduce active drug concentration before it reaches systemic circulation. Subcutaneous and intramuscular routes bypass the portal circulation entirely, explaining their superior bioavailability.

BPC-157 and the Oral Rodent Data

BPC-157 is frequently marketed as “orally active” based on rodent studies showing efficacy at 10 mcg/kg oral doses. These effects are reproducible in rats but the mechanism is not fully established — BPC-157 may act locally in the gut (NO pathway, EGF receptor), which would not require systemic absorption. No human pharmacokinetic data on oral BPC-157 absorption exists in peer-reviewed literature as of 2026.