How drugs work

Prescription Drugs Special Report

Ever wonder how the medications you take act in your body? This brief guide explains how several classes of popular drugs work.

Thousands of medications are in use today. Here are some of the most common mechanisms by which these drugs achieve their effects:

Antibiotic Drugs -- The development of safe and effective drugs to cure infections was arguably the most significant advance in drug development of the 20th century. Perhaps the most famous example is penicillin, which is derived from a toxin produced by the fungus Penicillium notatum that contaminated a laboratory experiment. The Scottish scientist Alexander Fleming noticed that the Penicillium fungus had done something to kill the bacterium Staphylococcus, which is responsible for many human infections.

Antibiotics have several modes of action. Penicillin disrupts the cell walls of bacteria, causing them to die. Some other antibiotics interfere with the ability of microorganisms to manufacture essential proteins or to reproduce.

Replacement-Therapy Drugs -- Some drugs work by replacing a substance the body lacks. For example, an iron supplement can correct iron deficiency, and daily insulin injections can treat diabetes. Another common example of a replacement-therapy drugs is a synthetic form of natural thyroid hormone (levothyroxine) that remedies the effects of a thyroid gland that has stopped working or was removed because of disease.

Drugs That Act on Enzymes -- Many body processes involve enzymes, which are proteins that facilitate biochemical reactions. An enzyme might, for example, bind to a molecule and break it down into smaller pieces, as occurs during digestion. Or an enzyme might build a larger molecule by joining small molecular building blocks.

Modulating the undesirable action of enzymes can correct disease processes. In fact, the top-selling drugs in the world, the cholesterol-lowering "statin" drugs, inhibit the action of a liver enzyme called HMG-CoA reductase. HMG-CoA reductase performs a key step in the biochemical assembly line that manufactures cholesterol. By blocking this enzyme, statin drugs foil the process, reducing blood levels of cholesterol -- particularly LDL, the "bad" cholesterol that raises the risk of heart disease.

Drugs That Act on Cell Receptors -- Cell receptors are structures on the surfaces of body cells. They are often compared to locks. The "key" that fits into the lock (a protein or some other molecule) is called a ligand. When the ligand binds to the surface of the receptor, forming a chemical bond, it signals the machinery inside the cell to do something.

For example, insulin binds to cell receptors and allows sugar (glucose) in the blood to enter cells. Some new drugs, such as the osteoporosis drug raloxifene (Evista), actually alter the shape of a receptor in ways that modify its action. Evista binds to the estrogen receptor, helping to prevent the bone loss associated with reduced estrogen.

Receptor-Blocking Drugs -- Sometimes known as antagonists, these drugs prevent the natural ligand "keys" from entering the cell receptor "locks," much as two people can't occupy the same seat in musical chairs.

In beta-blockers, an important class of cardiac drugs, the active ingredient is a molecule that prevents the hormone noradrenaline from binding to receptors in the heart. Blocking these "beta1" receptors slows the contractions of the heart muscle and makes them less forceful, which leads to reduced demand on the heart muscle and also lowers blood pressure.

Drugs That Alter Cell Transport -- There are still other ways to interfere with the transfer of molecular messages.

For example, the antidepressant drug fluoxetine (Prozac) is a selective serotonin reuptake inhibitor (SSRI). This drug works by preventing (or inhibiting) brain cells from collecting (or "re-uptaking") and subsequently processing for recycling used molecules of the brain messenger chemical known as serotonin. Thus, Prozac has the effect of increasing the amount of this messenger molecule available to brain cells, which helps to alleviate the symptoms of depression for many people.