Drug Absorption and Distribution Practice Test 2026 – Comprehensive Exam Prep

Subcutaneous absorption relies on the skin’s vascular and lymphatic networks. After injection, the drug sits in interstitial fluid and then moves into where it can cross into systemic circulation. Small, water‑soluble drugs typically diffuse into skin capillaries and enter the blood, which quickly carries them into the systemic circulation. Larger molecules or formulations made for slower release tend to enter lymphatic capillaries, which drain into the venous system via the thoracic duct, eventually delivering the drug systemically. This route also explains the slower onset and sometimes variable absorption with subcutaneous injections, compared with direct IV delivery, because absorption depends on tissue perfusion and lymphatic uptake rates. In short, the drug reaches systemic circulation through tiny capillaries and lymphatic vessels in the skin.