steady state
Steady state is the balance point reached when you keep taking a drug at regular intervals: the rate at which you put drug in finally equals the rate at which your body removes it. Like filling a leaky bucket at a constant pace, the water level first rises, then settles where inflow matches outflow. After that, the average drug level holds roughly constant from dose to dose.
Reaching steady state is not instant. With each dose the drug accumulates a little, and it takes about four to five half-lives of regular dosing before the level plateaus — regardless of dose size. The dose and frequency set how high the plateau is, but the half-life alone sets how long you must wait to get there.
Steady state is when a chronically dosed drug shows its true, stable behavior, so it is the condition under which therapeutic blood levels are usually judged and monitored. If a patient needs the effect immediately rather than days later, a one-time loading dose can be given to jump straight to the steady-state level.
A drug with a 24-hour half-life given once daily reaches steady state in about four to five days; a loading dose on day one lets the patient reach the target level immediately rather than waiting for accumulation.
A loading dose skips the wait for accumulation.
The time to reach steady state depends only on the half-life (about four to five of them), while the height of the steady-state level depends on dose, dosing interval and clearance.