You will never guess what the fifth and sixth best-selling prescription drugs are in the United States, so I’ll just tell you: Abilify and Seroquel, two powerful antipsychotics. In 2011 alone, they and other antipsychotic drugs were prescribed to 3.1 million Americans at a cost of $18.2 billion, a 13 percent increase over the previous year, according to the market research firm IMS Health.

Those drugs are used to treat such serious psychiatric disorders as schizophrenia, bipolar disorder and severe major depression. But the rates of these disorders have been stable in the adult population for years. So how did these and other antipsychotics get to be so popular?

Antipsychotic drugs have been around for a long time, but until recently they were not widely used. Thorazine, the first real antipsychotic, was synthesized in the 1950s; not just sedating, it also targeted the core symptoms of schizophrenia, like hallucinations and delusions. Later, it was discovered that antipsychotic drugs also had powerful mood-stabilizing effects, so they were used to treat bipolar disorder, too.

Then, starting in 1993, came the so-called atypical antipsychotic drugs like Risperdal, Zyprexa, Seroquel, Geodon and Abilify. Today there are 10 of these drugs on the market, and they have generally fewer neurological side effects than the first-generation drugs.