Idealized dose-response curves of three agonists (a, b, c) that increase heart rate but differ in potency, maximum effect, or both. Broken lines indicate 50% of maximum response (horizontal) and individual ED50 values (vertical).

80 beats per minute). However, the fact that the dose-response curve for drug a lies to the left of the curve for drug b indicates that drug a is more potent, that is, less of drug a is needed to produce a given response. The difference in potency is quantified by the ratio ED50b/ED50a: 3/0.3 = 10. Thus, drug a is 10 times as potent as drug b. In contrast, drug c has less maximum effect than either drug a or drug b. Drug c is said to have a lower intrinsic activity than the other two. Drugs a and b are full agonists with an intrinsic activity of 1; drug c is called a partial agonist and has an intrinsic activity of 0.5 because its maximum effect is half the maximum effect of a or b. The potency of drug c, however, is the same as that of drug b, because both drugs have the same ED50 (3 ^g /kg). The ED50 is the dose producing a response that is one-half of the maximal response to that same drug.

It is important not to equate greater potency of a drug with therapeutic superiority, since one might simply increase the dose of a less potent drug and thereby obtain an identical therapeutic response. Such factors as the severity and frequency of undesirable effects associated with each drug and their cost to the patient are more relevant factors in the choice between two similar drugs.

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