The effects of drugs vary according to the dose administered. The relationship between a given dose and the effects that this specific dose generates is called the dose-effect function (or dose–response curve). In many areas of pharmacology, dose-effect functions are monotonic (the functions move in only one direction), such as the relationship between an opioid dose and the response generated, which is typically analgesia (Figure 2.7). As the dose increases, pain relief increases – up to a point. At a certain level, called the asymptote, additional doses of the drug are unable to produce any additional effects. Dose–effect functions, however, can also be non-monotonic, such as the classic inverted U-shaped curve associated with locomotor activation produced by psychostimulant drugs (Figure 2.8). In such a dose-response function, the decreased effectiveness of cocaine/amphetamine in producing locomotor activity is attributable to an increase in stereotyped behavior.
Dose–effect functions describe two key characteristics of drug action: efficacy and potency. These two terms are often confused. Efficacy is the percentage of a maximum response. This can be seen on the Y-axis of the dose–response function in Figure 2.7. Potency is the dose required to produce a given effect relative to a standard. This can be seen on the X-axis of the dose-response function. Increases or decreases in effectiveness can be observed when comparing the levels reached by acetaminophen and morphine on the Y-axis in Figure 2.9. All three opioids shown in the figure are both more effective than acetaminophen (they produce a higher level of pain relief on the Y-axis). Further inspection of the figure also shows that the three opioids, although they have similar efficacy, have different potencies, reflected by parallel shifts of the dose to the right or left on the X-axis: codeine is less potent than morphine, and morphine is less potent than hydromorphone.