Drug Action

Drugs affect only the rate at which existing biologic functions proceed; they 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 function 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, and Alzheimer's 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, or cortisol. They can be used to replace hormones that are missing from the body.

Reversibility

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.

Affinity and Intrinsic Activity

A drug's action is affected by the degree of attraction (affinity) between it and its receptor on the cell's surface and, once it is bound to its receptor, by its ability to produce an effect (intrinsic activity). Drugs vary in their affinity and intrinsic activity.

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 they have little or no intrinsic activity, because their function is to prevent an agonist from interacting with its receptors.

Potency and Efficacy

A drug's effects can be evaluated in terms of strength (potency) or effectiveness (efficacy).

Potency 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 refers to the potential maximum therapeutic response that a drug can produce. For example, the diuretic furosemide eliminates much more salt and water through urine than does the diuretic chlorothiazide. Thus, furosemide has greater efficacy than chlorothiazide. However, greater potency or efficacy does not necessarily mean that one drug is preferable to another. When judging the relative merits of drugs for a patient, doctors consider many factors, such as side effects, potential toxicity, duration of effect (which determines the number of doses needed each day), and cost.