Pharmacokinetics: 1st order elimination

Basic, Basic Sciences

First order elimination occurs when a constant proportion of drug is eliminated per unit time. The rate of elimination is proportional to the amount of drug remaining (at higher concentrations, more drug is eliminated per unit time). Elimination processes are not saturated but rather can accommodate increased concentrations. This is true of most drugs. In contrast, zero order processes are those on which elimination is a constant quantity per unit time, independent of total drug concentration.

When concentration is graphed as a function of time, the relationship is exponential. If instead the natural log of the concentration is plotted on the Y-axis the graph is linear with a slope of –k.

1st order and 0 order elimination at


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Elizabeth Kim, MD