Learn pharmacokinetics
Steady state: when levels stop rising
Medically reviewed by Javeria Muneer, Pharmacist · Last reviewed:
When a drug is given repeatedly, each new dose arrives before the previous one has fully cleared. The leftover adds to the new dose, so peaks and troughs climb with every interval. Eventually the amount eliminated in one interval equals the amount given, and the curve settles into the same repeating shape. That is steady state.
How long it takes
Time to steady state depends only on half-life, not on the dose or the interval. After one half-life you are at 50% of steady state, after three at about 88%, and after five at about 97%. A drug with a 6-hour half-life reaches steady state in about a day and a half; one with a 48-hour half-life takes over a week.
This is why some medicines take days to reach full effect, and why a loading dose is sometimes used to get there faster.
Accumulation ratio
The accumulation ratio R = 1 / (1 − e^(−k·τ)) tells you how much higher steady-state levels are than after the first dose. When the dosing interval is long compared with the half-life, R is close to 1 and there is little build-up. When doses are frequent relative to half-life, R is large and levels rise substantially.
Average steady-state concentration equals F × Dose / (CL × τ). Doubling the dose doubles it; halving clearance also doubles it.
See it in the simulator
Add more doses or shorten the interval to watch the peaks climb towards a plateau.
