Antidepressants are drugs used to treat depression—and certain other mental disorders such as attention deficit disorder, obsessive-compulsive disorder, narcolepsy, anxiety, and panic disorder--caused by chemical imbalances in the brain. Antidepressants, which work by rebalancing these chemicals called "neurotransmitters," are grouped into three primary categories based on their mechanisms of action: tricyclics (or heterocyclics), monoamine oxidase inhibitors, and selective serotonin re-uptake inhibitors. Three drugs, Wellbutrin, Desyrel, and Effexor, differ in their mechanisms of action from other antidepressants and from each other. Antidepressants are available only by prescription, take from four to six weeks to take full effect, work only on treatable disorders, are non-addictive, and are effective for approximately 80% of individuals treated.

When clinical depression and many other mental illnesses were determined to be primarily biological rather than psychological, development of synthetic drugs revolutionized the treatment of mental disorders. Tricyclics (TCAs), introduced into the marketplace in the...