Evolution of plasma concentrations of a drug after repeated administrations
When the interval of time separating two successive administrations of a drug is sufficiently long, at the time of the second administration, the drug is not present any more in the body.
The three factors to take into account in the case of repeated administrations of a drug are the administered dose, the frequency of administrations and the half-life of the drug.
Short half-life, not very frequent administrations
The duration of persistence of a drug in the body increases with the increase of dose and half-life. When the interval between two successive administrations is long, there is no accumulation of the drug.
Plasma concentration of a drug after repeated administrations without accumulation
Long half-life or frequent administrations
When the frequency of administration of a drug is sufficient or when its half-life is sufficiently long, there is a residual concentration of the drug at the time of its later administration. In this case, the repetition of its administrations at constant intervals leads to a progressive increase in its concentration until reaching a maximum concentration or steady state which is reached in approximately five half-lives. The administered dose remains constant whereas the quantity of product eliminated by unit of time rises, until compensating for the quantity supplied by each administration.
Plasma concentration of a drug after repeated administrations with accumulation - Min = residual concentration