Drug Response and Metabolism

The fact that different people react differently to the same relative drug dosage is an important and often critical aspect of pharmacology. Two patients who are given the same drug may exhibit different magnitudes of a beneficial response as well as different adverse effects. Several primary factors that are responsible for variations in the response to drugs are discussed below. 

FIGURE 3-4 Relationship between dosing interval and plasma concentrations of the antiasthmatic drug theophylline. A constant intravenous infusion [shown by the smoothly rising line] yields a desired plasma level of 10 mg/L. The same average plasma concentration is achieved when a dose of 224 mg is taken every 8 hours, or a dose of 672 mg every 24 hours. However, note the fluctuations in plasma concentration seen when doses are taken at specific hourly intervals. 

In contrast to an additive effect, drugs with opposite actions may essentially cancel each other out, thus negating or reducing the beneficial effects of one or both medications. A drug that causes bronchodilation (i.e., for the treatment of asthma) will be negated by an agent that constricts the bronchioles. 

Some of the most serious problems occur during drug interactions because one drug delays the biotransformation of the other. If a second compound inhibits the enzymes that 

Children are also subject to problems and variability in drug metabolism.54 Because liver 

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