Clearance, half-life & steady state
What it means
Steady state in ~5 half-lives; same to wash out after stopping. Loading dose = Vd × target concentration (gets there immediately); maintenance = clearance × target. Long-t½ drugs (amiodarone ~58 days, fluoxetine/norfluoxetine) take weeks to reach steady state and to clear — explaining persistent interactions after stopping.
Worked example
A loading dose of digoxin/amiodarone is used because waiting 5 half-lives for effect is unacceptable.
In the exam
To achieve a therapeutic level immediately rather than wait several half-lives, a clinician gives a dose calculated from the drug's volume of distribution and target concentration.
What settles it
Loading dose depends on Vd (fills the compartment); maintenance dose depends on clearance (replaces what's eliminated).
Classically confused with
First-order vs zero-order kinetics
Source: StatPearls — Pharmacokinetics