drugs
Drug Action
Potency, Efficacy, and Effectiveness of Drugs
A drug’s effects can be evaluated in terms of potency, efficacy, or effectiveness. Potency(strength) refers to the amount of drug (usually expressed in milligrams) needed to produce an effect, such as relief of pain or reduction of blood pressure. For instance, if 5 milligrams of drug A relieves pain as effectively as 10 milligrams of drug B, drug A is twice as potent as drug B. Efficacyis a drug's capacity to produce an effect (such as lowering blood pressure). For example, the diureticfurosemideeliminates much more salt and water through urine than does the diuretichydrochlorothiazide. Thus,furosemidehas greater efficacy thanhydrochlorothiazide. Effectivenessdiffers from efficacy in that it takes into account how well a drug works in real-world use. Often, a drug that is efficacious in clinical trials is not very effective in actual use. For example, a drug may have high efficacy in lowering blood pressure but may have low effectiveness because it causes so many side effects that people take it less often than they should or stop taking it entirely. Thus, effectiveness tends to be lower than efficacy. Greater potency, efficacy, or effectiveness does not necessarily mean that one drug is preferable to another. When judging the relative merits of drugs for a person, doctors consider many factors, such as side effects, potential toxicity, duration of effect (which determines the number of doses needed each day), and cost. 