Following repeated drug administrations, the systemic concentrations in the body reach an equilibrium where the rate of drug intake equals the rate of excretion. This equilibrium is described as steady-state. Typically, it takes administrations over approximately five half-lives of the drug to reach steady-state independent of dose level and dosing interval. We can calculate steady-state concentration (Css) by dividing the total exposure over the dosing interval (AUClast) by the interval time (Tlast).