M06.01.006 Absorption

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Learning Objective

Describe the key determinants of drug absorption, define pharmacokinetic parameters related to absorption (Cmax, tmax, lag time, onset of action, duration of action), and explain how intravascular vs. extravascular administration affects bioavailability.

Absorption

Absorption refers to the process by which a drug moves from the site of administration into the systemic circulation. All factors that affect drug permeation across membranes directly determine drug absorption.

Intravascular administration (IV):

Absorption does not occur.
No loss of drug during entry.
Bioavailability = 100%.

Extravascular routes (PO/oral, IM, SC, inhalation)

Drugs must cross biological membranes → subject to first-pass metabolism, incomplete absorption, and variable bioavailability.
Bioavailability < 100%.

Pharmacokinetic Terms Related to Absorption

Parameter	Definition
Cmax	Maximum plasma drug concentration is achieved after administration.
tmax	Time at which Cmax occurs. Reflects the rate of absorption.
Lag time	Time from administration until the drug first appears in the blood.
Onset of action	Time from administration until plasma level reaches the minimum effective concentration (MEC).
Duration of action	Time during which plasma drug levels remain above MEC.
Time to peak	Time from administration to Cmax (same as tmax).