The drug Prozac was introduced in 1987. By 1990, psychiatrists in the United States prescribed Prozac more often than any other drug. Five years later, its maker, Eli Lilly and Co., reported sales of over $2 billion. Originally designed as an anti-depressant, Prozac has since been used to combat obesity, shyness, obsessive-compulsive behavior, and even back pain. It improves mood, energy levels, optimism, and feelings of inadequacy. Not only has Prozac helped millions of people deal with their emotional and psychological problems, but it has changed their understanding of moods and personality.

Prozac is one of a group of "designer" drugs known as Selective Serotonin Reuptake Inhibitors (SSRIs). These drugs work by increasing levels of the chemical serotonin (seh-reh-TOE-nin) in the brain. Older antidepressants are less specific. They change the balance of a number of brain chemicals in the hope that one of them will have the desired effect. In theory...