Selectivity vs Specificity
It is important for prescribers to remember that selectivity for a receptor subtype is only a relative concept: selectivity does not equate with specificity.
Agonist or antagonist drugs that are considered to be 'selective' for one receptor subtype can still produce significant effects at other subtypes if a large enough dose is given. This is particularly important if the beneficial effects are activated by one receptor subtype and the unwanted effects by another.
Click here for an important clinical example.
Selectivity is useful in clinical practice only when the ratio of the impact of the drug at the two receptor sites is 100 or more. When the ratio is lower than this, it is difficult to predict drug doses that will exploit the difference in subtype activity.
Selectivity is most likely to be achieved at the lowest effective dose. Higher doses will not increase efficacy and will begin to have effects at other receptor sites.
