First-order vs zero-order kinetics
What it means
First-order: elimination rate proportional to concentration → constant half-life (most drugs). Zero-order: a fixed amount eliminated per unit time (enzymes saturated) → no constant half-life; small dose increases cause disproportionate level rises. Zero-order drugs: phenytoin, ethanol, high-dose aspirin/salicylates.
Worked example
A small phenytoin dose increase causes a large, sometimes toxic, level rise because elimination is saturable (zero-order).
In the exam
A patient becomes toxic after a modest dose increase of an antiepileptic whose elimination is saturable, so plasma levels rise disproportionately.
What settles it
Zero-order = saturable, disproportionate level rises (phenytoin/ethanol/aspirin); first-order = proportional, predictable half-life.
Classically confused with
Clearance, half-life & steady state
Source: StatPearls