First-order kinetics vs Zero-order kinetics
How to tell First-order kinetics and Zero-order kinetics apart when a board question makes them look alike.
First-order: a constant fraction cleared per unit time, constant half-life; zero-order: a constant amount cleared, half-life rises with dose.
Side by side
A constant fraction of drug is eliminated per unit time; elimination rate rises as concentration rises.
A constant amount of drug is eliminated per unit time because elimination pathways are saturated.
Clearance capacity is not saturated, so concentration drives the elimination rate.
Metabolic or transporter capacity is saturated, so elimination cannot increase proportionally with concentration.
Most drugs follow first-order kinetics over therapeutic ranges; half-life stays relatively constant.
Ethanol, phenytoin, or high-dose salicylate-style saturation clues point zero-order behavior.
The trap
Zero-order does not mean no elimination; it means capacity-limited fixed-rate elimination.
The repair move
Convert every pharmacokinetics question into fraction-per-time versus amount-per-time before naming the answer: (1) constant half-life regardless of dose equals first-order; constant rate of elimination regardless of dose equals zero-order; (2) on a concentration-time plot, exponential decay (straight line on semi-log axes) equals first-order; linear decay (curved on semi-log axes) equals zero-order; (3) memorize the PEA drugs (Phenytoin, Ethanol, Aspirin at toxic doses) as the classic zero-order examples they can become toxic quickly because each dose increment causes an exponential rather than proportional rise in level.
Try a free question
CoreA drug is eliminated at a constant fraction per unit time; as the concentration rises, the amount eliminated per hour rises proportionally. Which kinetic model is this?
Show answer and explanation
Answer: A. First-order kinetics. First-order elimination removes a constant fraction per unit time and is concentration dependent.
Why the other choices are tempting
- B. Zero-order elimination removes a constant amount per unit time after pathways are saturated.
- C. Saturable receptor binding is pharmacodynamics, not elimination kinetics.
- D. Phase II metabolism describes conjugation reactions.
- E. High volume of distribution describes tissue distribution, not elimination rate pattern.
Flashcards
Tap a card to flip it.
What is the fastest separator for First-order kinetics vs Zero-order kinetics?
First-order: a constant fraction cleared per unit time, constant half-life; zero-order: a constant amount cleared, half-life rises with dose.
What clue should make you choose First-order kinetics?
A constant fraction of drug is eliminated per unit time; elimination rate rises as concentration rises.
Quick answers
What is the fastest way to tell First-order kinetics from Zero-order kinetics?
First-order: a constant fraction cleared per unit time, constant half-life; zero-order: a constant amount cleared, half-life rises with dose.
What is the key clue for First-order kinetics?
A constant fraction of drug is eliminated per unit time; elimination rate rises as concentration rises.
What is the key clue for Zero-order kinetics?
A constant amount of drug is eliminated per unit time because elimination pathways are saturated.
What is the common trap with First-order kinetics vs Zero-order kinetics?
Zero-order does not mean no elimination; it means capacity-limited fixed-rate elimination.