3. Clinical pharmacokinetics: application of the parameter "terminal half life" of drugs in clinical practice.
Explains the terminal elimination half-life as the time for plasma concentration to halve, and what lengthens or shortens it, such as changes in volume of distribution and clearance. It gives a table of approximate half-lives for example drugs, from dobutamine to digitoxin. It then covers practical uses: time to steady state, washout after stopping, dosing intervals and loading doses. A comparison table contrasts first-order and zero-order kinetics.
Inside: 4 sections · 3 tables · 1 formula · 15 flashcards · 2 min
Topic 3 of 30 in the MU Sofia syllabus for Clinical Pharmacology (Medicine, Year 5). The free AceMed note covers it in 4 sections, about 176 words (2 min of reading), with 15 flashcards.
In this note
Introduction
What determines t½
Clinical importance
First-order versus zero-order kinetics
Introduction
The terminal elimination half-life (t½) is the time needed for the plasma concentration to fall by half. In first-order (linear) kinetics it is constant whatever the concentration.
What determines t½
Half-life is a derived parameter: it lengthens when Vd rises (e.g. obesity for lipophilic drugs) or clearance falls (kidney or liver failure, old age, enzyme inhibitors), and shortens with enzyme induction.
Examples
Clinical importance
Time to steady state: about 4–5 half-lives of regular dosing (94–97% of steady state).
Time to eliminate the drug after stopping: also 4–5 half-lives.
Duration of action and therefore the dosing interval: short t½ drugs need frequent doses, infusion or modified-release forms; long t½ drugs can be given once daily.
Whether a loading dose is needed: for long t½ drugs (digoxin, amiodarone), waiting 4–5 half-lives for steady state would take days or weeks.
Wash-out before switching drugs or before surgery; persistence of adverse effects after stopping.
First-order versus zero-order kinetics
Half-life tells you three practical things: how often to dose, how long to wait for steady state, and how long the drug stays after you stop it (each about 4–5 half-lives).
Read the full note free
The whole note, all 4 sections, is free with an AceMed account, with its 15 flashcards. Premium adds flashcard recall, highlights and Ask AI.
