First-Pass Metabolism
The breakdown of an orally administered drug by the gut and liver before it reaches systemic circulation — the reason most peptides are injected.
Definition
When a drug is swallowed, it's absorbed into the portal circulation and processed by the liver before reaching the rest of the body. For peptides, digestive enzymes in the stomach and gut largely destroy the molecule outright, and what survives is typically degraded by hepatic enzymes. Subcutaneous injection bypasses both steps and is the default route for clinically relevant peptide dosing.
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Bioavailability
The fraction of a dose that reaches circulation in active form — a key reason most peptides are injected subcutaneously rather than taken orally.
Delivery & DosingSubcutaneous InjectionSQ Injection
Injection into the fatty layer just below the skin — the standard route for peptide dosing, self-administered with an insulin syringe.
Delivery & DosingOral Bioavailability
The fraction of an orally taken drug that survives digestion and reaches circulation — typically under 1% for peptides.
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