Studies have shown that low levels of serotonin, a mood- regulating hormone, can cause depression. Antidepressants known as selective serotonin reuptake inhibitors (SSRIs) work by slowing down the reabsorption of serotonin into the neural system, thereby increasing the serotonin level in the brain. Thirty-five million people have used Prozac, the most prescribed SSRI, and its effects have been frequently praised. In a book about the experiences of SSRI users, Janet Thacker writes that since taking Prozac: “I smile, and I go out of my way to meet people and make friends.” However, some critics wonder if Prozac has a dangerous effect on personality.

One incident that inspired debate was a 1989 mass shooting at a Kentucky factory. Joseph Wesbecker shot twenty coworkers, killing eight, before killing himself. Wesbecker had been taking Prozac for a month. Survivors and relatives of the...