The definition of half-life (t 1/2) is the time required for the concentration to fall to 50% of its current value. This final equation shows the relationship between t 1/2 and the elimination rate constant. To do this we are looking for a time = t 1/2 in which Using this equation, we can determine when half of the drug has left the body, which is also known as the half-life of the drug. To understand how this relates to half-life requires a bit more math. The rate at which drug is eliminated from the body is in the units of inverse time (e.g.
One of the secondary pharmacokinetic parameters is the elimination rate (k el), which is calculated as:
By combining these primary parameters, we can generate secondary pharmacokinetic parameters. These primary parameters are volume of distribution, clearance, bioavailability, and absorption rate. If you remember the discussion on primary pharmacokinetic parameters, these parameters depend on the physiology of the body and the physiochemical properties of the drug. The third statement is never true! To understand half-life, and what it means, we need to talk about where it comes from. The first two statements are true under specific conditions. Half-life is a primary pharmacokinetic parameter.Because the drug has a long half-life, you don’t need to dose very frequently.If you double the dose, you will increase the duration of effect by 1 half-life.Here are some common statements that use half-life: And when the assumptions are false, the derived interpretations are also incorrect. The reason for the multitude of errors with half-life is that in most cases, the assumptions required to use half-life correctly are rarely valid. One of the most incorrectly used but most often quoted parameters is half life.
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