Most medications given to patients have a direct effect on a particular and often specific molecule or class of molecules. These molecules are likely to be proteins serving as enzymes to catalyze chemical reactions, or as receptors, ion channels, or transport molecules. Other common sites of action include direct binding to nucleic acids. Some useful medications have less “interesting” sites of action, especially those drugs that do not enter the body. For example, sun-blocking creams stay on the surface of the skin to physically block UV rays in sunlight and have no specific molecular site of action. Antacid tablets (e.g., magnesium hydroxide or calcium carbonate) chemically buffer the HCl acid in the stomach, but could just as easily buffer other acids; this is hardly a specific molecular target of action.

Once a drug interacts with its target molecule, however, its pharmacologic effects then can become obvious at other levels. Interaction of a drug with its molecular target then has effects on the cell, subsequently on a tissue, eventually on an organ system, and ultimately, on the intact organism (or patient in clinical pharmacology). In fact, a further level of action might be on the patient's community. For example, the use of vancomycin in one hospitalized patient can have an effect on the broader hospital community by helping to increase the development of Staphylococci resistant to vancomycin within that hospital environment.

Thus the question “What does terazosin do?” might be answered by saying that the drug acts as an inhibitor of alpha-1 adrenoceptors (at the molecular level), thereby decreasing the influx of calcium into smooth muscle cells; thereby relaxing the smooth muscle tissue at the bladder neck and prostate; thereby facilitating bladder emptying and increasing rate of urine flow (at the system level); and thereby decreasing complaints of poor urine flow, frequency, dribbling, or nocturia (at the level of the 68-year-old man with bladder outlet obstruction due to benign prostatic hypertrophy with some component of reversibility). When treating patients with drugs, it is important to keep all of these levels of drug action in mind.