n. an atypical antipsychotic of the benzisoxazole class. It was the second atypical antipsychotic introduced into the U.S. market (clozapine was the first). It has a less frequent incidence of extrapyramidal symptoms than do conventional antipsychotics when used at a lower dose range, and it acts as a potent inhibitor of both D2 dopamine and 5-HT2 serotonin receptors. It is also approved for bipolar disorder–related acute manic or mixed episodes and for irritability in autism. U.S. trade name: Risperdal.