Abstract :Potency is determined by the dose of the drug required to produce an effect. A highly potent drug (e.g., fentanyl, estrogen, calcitriol) evokes a desires pharmacological response in target population of patients at low concentrations. On the other hand, these drugs may also evoke unintended, adverse effects at doses lower than the intended effects. These effects may be acceptable in the patient population; however they are not acceptable in other populations, such as workers, or patients who would receive such a drug as a contaminant. Due to these differences, defining an HPAPI is not uniform.