[The use of the concepts of "half-life" and "clearance" in practice and in pharmacokinetics].

The elimination of drugs from the body and the different modes of dealing with this process on a quantitative scale are discussed. The concepts of half-life and clearance are briefly reviewed. It is shown that with the term "half-life" the investigator focuses upon the elimination of the drug from the whole of the body. With the term "clearance" the interest is focused on the drug-clearing organs or on the effectiveness of these organs in eliminating the drug from the blood stream. As a consequence of this situation, the half-life is the truly useful parameter for the physician since it enables him to establish appropriate drug administration schedules for his patients. Clearance values, on the other hand, are chiefly of use to the pharmacokineticist in establishing the kinetic profile of a new drug.