This learning model is currently available in English.
Explore the system
Repeated doses accumulate
Vary interval and half-life. See each dose add to what remains, producing peaks, troughs, and a repeating plateau.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 1 · Repeated doses accumulate · PDF 19 · 1.11
Course notes and existing practice
Learning objective
Explain accumulation and washout without confusing an approximate plateau or small residual with an exact endpoint.
Repeated input and a repeating pattern
The therapeutic-window diagram contrasts concentrations associated with intended effects and concentrations associated with unacceptable effects. With stable repeated input and first-order elimination, accumulation approaches a repeating concentration pattern. At steady state, average input and elimination balance; peaks and troughs can still occur. The course uses about six half-lives as its plateau approximation. That is a chosen closeness to an asymptote, not a moment when the curve becomes exactly flat.
How much remains?
In the same idealized model, five half-lives leave 1/32, or 3.125%, of the starting level; six leave 1/64, or 1.5625%. With constant input from zero, the corresponding approach to steady state is 96.875% and 98.4375%. After input stops, residual drug and active metabolites can persist. These percentages do not determine a safe medication-switching interval or imply that the body contains exactly zero drug.
- Steady state
- A stable repeating exposure pattern in which average drug input and elimination balance.
- Washout
- The decline in drug exposure after input stops; active metabolites may have different time courses.
Professor deck 04, PDF pages 46, 47, 48, 49. 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
A repeating steady state can contain peaks and troughs. Average input and elimination balance over an interval; elimination has not stopped. Keep this concentration-time idea separate from Chapter 3’s therapeutic index, which is a ratio between specified population endpoints.
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.