Drugs affect only the rate at which existing biologic functions proceed. (See also Definition of Drug Dynamics.) Drugs do not change the basic nature of these functions or create new functions. For example, drugs can speed up or slow down the biochemical reactions that cause muscles to contract, kidney cells to regulate the volume of water and salts retained or eliminated by the body, glands to secrete substances (such as mucus, stomach acid, or insulin), and nerves to transmit messages.
Drugs cannot restore structures or functions already damaged beyond repair by the body. This fundamental limitation of drug action underlies much of the current frustration in trying to treat tissue-destroying or degenerative diseases such as heart failure, arthritis, muscular dystrophy, multiple sclerosis, Parkinson disease, and Alzheimer disease. Nonetheless, some drugs can help the body repair itself. For example, by stopping an infection, antibiotics can allow the body to repair damage caused by the infection.
Some drugs are hormones, such as insulin, thyroid hormones, estrogens, or cortisol. They can be used to replace natural hormones that are missing from the body.
Most interactions between a drug and a receptor or between a drug and an enzyme are reversible: After a while, the drug disengages, and the receptor or enzyme resumes normal function. Sometimes an interaction is largely irreversible, and the drug’s effect persists until the body manufactures more enzyme. For example, omeprazole, a drug used in the management of gastroesophageal reflux and ulcers, irreversibly inhibits an enzyme involved in the secretion of stomach acid.
A drug’s action is affected by the quantity of drug that reaches the receptor and the degree of attraction (affinity) between it and its receptor on the cell’s surface. Once bound to their receptor, drugs vary in their ability to produce an effect (intrinsic activity). A drug's affinity and intrinsic activity are determined by its chemical structure.
Drugs that activate receptors (agonists) must have both great affinity and intrinsic activity: They must bind effectively to their receptors, and the drug bound to its receptor (drug-receptor complex) must be capable of producing an effect in the targeted area. In contrast, drugs that block receptors (antagonists) must bind effectively but have little or no intrinsic activity because their function is to prevent an agonist from interacting with its receptors.
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.
Efficacy is a drug's capacity to produce an effect (such as lowering blood pressure). For example, the diuretic furosemide eliminates much more salt and water through urine than does the diuretic hydrochlorothiazide. Thus, furosemide has greater efficacy than hydrochlorothiazide.
Effectiveness differs 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.