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. Depending on the degree of tolerance or resistance that develops, a doctor may increase the dose or select an alternative drug. Drug Design and Development Many of the drugs in current use were discovered by experimental trial and observation in animal and human subjects. Newer approaches to drug development are based on a determination of the abnormal biochemical and cellular changes caused by disease and the design of compounds that may specifically prevent or correct these abnormalities. When a new compound shows promise, it's usually modified many times to optimize its selectivity, potency, receptor affinity, and therapeutic efficacy.