steady state
Steady state is the balance point reached during regular repeated dosing, where the rate of drug going in equals the rate of drug leaving. Picture a sink with the tap running and the drain open: at first the water level rises, but eventually inflow and outflow match and the level holds steady. The plasma concentration then plateaus, oscillating gently around an average value between doses.
It is not reached instantly. Because elimination is proportional to how much drug is present, the body removes drug faster as the level climbs, until removal catches up with input. This takes about 4–5 half-lives regardless of how often you dose — a fact that lets you predict when a regimen will reach its plateau.
The height of the plateau depends only on the dosing rate and clearance, not on how quickly you got there. Giving the same daily amount in smaller, more frequent doses reaches the same average steady-state level but with smaller peak-to-trough swings. A loading dose can jump straight to the target plateau without waiting the usual 4–5 half-lives.
A drug with a 24-hour half-life dosed once daily reaches steady state in about 4–5 days; until then, judging the dose from blood levels would be premature.
Steady state arrives after ~4–5 half-lives.
Time to steady state depends only on half-life (≈4–5 half-lives); the height of the plateau depends only on dosing rate and clearance. The two are independent.