This learning model is currently available in English.
Explore the system
How much remains?
Change amount, half-life, and elapsed time. Compare a constant fractional loss with a constant absolute loss.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 1 · How much remains? · PDF 18, 19 · 1.10
Course notes and existing practice
Learning objective
Calculate successive halvings in a first-order model and distinguish plasma decline from effect duration.
Fraction versus amount
In a first-order elimination model, a constant fraction is removed per unit time. Half-life is the time for the modeled concentration to decline by half. After n half-lives, the remaining fraction is (1/2)^n. Zero-order behavior instead removes a roughly constant amount per unit time over the relevant range. The lecture's alcohol slogan is not a reliable clock for impairment or safety.
Concentration is not the whole effect
Absorption can continue while elimination occurs, and redistribution can move drug away from blood or a target tissue. An early fall in plasma concentration need not be entirely chemical destruction or excretion. Half-life and duration of an effect therefore cannot be treated as interchangeable. The simple halving exercise assumes no new input and a suitable first-order phase.
- Half-life
- The time required for a concentration to fall by half in the specified kinetic phase.
- Redistribution
- Movement of drug between tissue compartments after its initial distribution.
Professor deck 04, PDF pages 44, 45. Course material is paraphrased. Qualifications and excluded clinical generalizations are documented in the Pharmacokinetics editorial record. Source review does not establish clinical validation.
Examples and practice explanations are original study aids, not professor quotations, exam requirements or medication instructions.
Julien connection: a distinction to explain on your own
Julien describes approximately five to six half-lives to approach steady state; the course emphasizes approximately six. In the first-order model, six half-lives leave 1.5625% of the starting amount. Near-complete elimination is not literal zero, and early redistribution can lower plasma levels without equivalent bodily elimination.
Book connection: see the Julien Chapters 1–4 · Midterm study section in the Library for worked examples and recall practice. The cited source uses consolidated PDF page numbers.