Drug Absorption by Route: From Fastest (IV) to Slowest (Topical)

Absorption Speed Ranking

1. Intravenous (IV) – 100% bioavailability; immediate onset.
2. Inhalation (Pulmonary) – Rapid alveolar absorption; seconds to minutes.
3. Sublingual (SL) – Direct mucosal absorption; bypasses first-pass metabolism.
4. Transdermal (TD) – Controlled release through skin; steady state over hours/days.
5. Rectal (PR) – Partial first-pass bypass; faster than oral.
6. Oral (PO) – GI absorption; subject to first-pass metabolism; variable bioavailability.
7. Buccal (BC) – Cheek mucosa; similar to sublingual but slower.
8. Topical (TP) – Local skin effect; minimal systemic absorption.

Modifying Factors

Drug lipophilicity, molecular size, formulation (e.g., liposomes), pH, blood flow, disease state, and patient age all influence actual absorption rate.