Accumulation and Steady State Concentration

Sometimes drugs are taken just once, for example, to relieve anxiety caused by an exasperating event. Often it is necessary, however, to have the drug present in the brain for prolonged periods of time. It is usually assumed beneficial to maintain steady concentrations, and the drug administration regime must support this aim. The drug must be administered repeatedly according to a schedule. In the beginning of repeated administration, drug levels will increase with each dose taken because it is normally important to take a new dose before the concentration in the blood has fallen to zero. Thus the drug accumulates in the body. The aim is to achieve a persistent, or steady-state, concentration in the blood (and at the site of action) with only limited fluctuations. Drug half-life is again an important measure. For example, if the second dose of a drug is given when one half-life of the drug has passed, the concentration of the drug will increase to 150 percent of what was observed after the first dose. Continuing the same treatment schedule would, however, cause a further increase, which becomes ever slower until the steady state (in our example, 200 percent of the initial peak level) is achieved. Knowing the drug half-life, it is possible to design a treatment schedule with minimal fluctuations in plasma levels.

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