where t1/2 is the half-life and ke is the elimination rate constant calculated from the slope of the declining portion of the concentration-time profile (Fig. 5.3). By definition, half-life denotes that 50% of the drug in the body at a given time will be eliminated over the calculated period. However, this does not mean that the same amount of drug is eliminated each half-life. For example, Figure 5.3 shows that during the first half-life period (0-5 hours) the drug concentration is reduced from 100 ^g/mL to 50 ^g/mL. However, during the second half-life period (5-10 hours), even though the amount in the body is reduced by 50%, the concentration falls only from 50 ^g/mL to 25 ^g/mL (a reduction in concentration of 25 ^g/mL). This concept is also illustrated in Table 5.1. It takes approximately five half-lives for 97% of the drug to be eliminated from the body (regardless of the duration of the half-life). Thus, if one wished to switch a patient from one drug to another but not have both drugs present in substantial quantities,
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