Schizophrenia is characterized by both positive and negative symptoms with positive symptoms such as hallucinations. The mesolimbic hypothesis has been the main theory on what causes psychotic symptoms, based on the function of typical antipsychotics, which have high binding affinities for DA D2 receptors, especially in the mesolimbic system. Clozapine, the first atypical AP, was introduced in 1973 and was highly effective with the bonus of decreased frequency in extrapyramidal side effects. These multi receptor drugs are named atypical AP’s. The atypical AP’s as a group demonstrate high affinity for serotonin 5-HT2 receptors and less affinity for D2 receptors with selectivity for the mesolimbic DA system instead of the nigrostriatal regions. These properties among other receptor functions are believed to be the reason for reduced extrapyramidal side effects and improved efficacy.