For example, the drug lovastatin, used to treat some people who have high blood cholesterol levels, inhibits the enzyme HMG-CoA reductase, which is critical in the body's production of cholesterol. Affinity and Intrinsic Activity Two drug properties important to a drug's action are affinity and intrinsic activity. Affinity is the mutual attraction or strength of the bond between a drug and its target, whether it's a receptor or an enzyme. Intrinsic activity is a measure of the drug's ability to produce a pharmacologic effect when bound to its receptor. For instance, if 5 milligrams of drug B relieves pain as effectively as 10 milligrams of drug A, then drug B is twice as potent as drug A. Greater potency does not necessarily mean that one drug is better than another.
Efficacy refers to the potential maximum therapeutic response that a drug can produce.
For example, the diuretic furosemide eliminates much more salt and water through the urine than does the diuretic chlorothiazide. Thus, furosemide has a greater efficacy, or therapeutic effectiveness, than chlorothiazide. As with potency, efficacy is only one factor that doctors consider when selecting the most appropriate drug for an individual patient. Tolerance occurs when the body adapts to the continued presence of the drug. The term resistance is used to describe the situation in which a person no longer responds well to an antibiotic, antiviral, or cancer chemotherapeutic drug.