The magnitude of a physiological/pharmacological response obtained from optimal receptor occupancy by a drug reflects its efficacy. The efficacy of a drug is related to its chemical structure and is predicated on the drug’s intrinsic activity. The graded dose-response relationship is the quantification of a specific response elicited by a drug over a range of dosages. It is expressed visually and mathematically by a dose-response curve (Figure 3).

Figure 3.
Hydrogen and ionic bonds
The dose-response curve is established by placing the logarithmic value for dosage on the x-axis and the quantified response on the y-axis.

The slope of the dose-response curve reflects the effect of a drug associated with increasing dosages. The dose of a drug that produces maximal effect (Emax) is the ceiling dose of that drug. For example, the analgesic ceiling dose of ibuprofen is 800 mg three times a day (i.e., a total daily dose of 2400 mg); the administration of higher daily doses of ibuprofen would not provide any additional therapeutic (analgesic) benefit and may increase toxicity.