Clozapine was introduced into the U.S. market in 1990 as a novel atypical antipsychotic drug. Before that, the last atypical antipsychotic drug approved by the Food and Drug Administration (FDA) was loxapine, way back in 1975. Since 1990 three more atypical antipsychotics have been introduced—risperidone in 1994, olanzapine in 1996, and quetiapine in 1997. As of April 1998 one more, ziprasidone, is under FDA review. Still more are in various stages of development. The promise of atypical antipsychotic drugs is that they have minimal or no extrapyramidal side effects, effectively treat negative symptoms, do not elevate serum prolactin in long-term administration (except for risperidone), and have enhanced efficacy for treatment-refractory schizophrenia (proven only in clozapine use).