Subcutaneous Injection (SC)

Subcutaneous injection deposits a solution into the loose connective and adipose tissue below the dermis and above the muscle. Absorption follows two routes, and molecular size decides the split: smaller molecules are taken up mainly by local capillaries, while larger ones travel through lymphatic vessels and reach the blood only after passing through lymph nodes. Most therapeutic peptides sit in the capillary-dominated range, yet their absorption is still slow enough that it, rather than elimination, often sets the shape of the concentration curve.

It is the default route for self-administered peptides because it needs only a short, fine needle and tolerates the small volumes peptide potency allows, conventionally a millilitre or two before the tissue objects. Absorption rate varies by site, abdominal tissue generally being faster than thigh or buttock for insulin, and with local blood flow. Bioavailability is high for many peptides but rarely complete, since some fraction is degraded by peptidases in the interstitium or lost in lymphatic transit.

Against intravenous administration, the route trades an immediate and fully known input for a blunted peak, a delayed time to maximum concentration and an absorption step with its own variability. That blunting is often the therapeutic point, since it flattens the concentration profile without a formulation change.

The mistaken inference is that subcutaneous means fully absorbed, so an injected amount equals a systemic amount; incomplete bioavailability by this route is normal and compound-specific. A second error is assuming an accidental intramuscular injection is merely a deeper version of the same thing. Muscle is better perfused, so absorption is faster and the peak higher, a different exposure profile from the one characterised for the product.