ABSTRACT: BACKGROUND:Different effectiveness profiles among antipsychotics may be a key point to optimize treatment in patients suffering a first episode of psychosis to impact on long-term outcome. The aim of this study is to compare the clinical effectiveness of olanzapine, risperidone, haloperidol, aripiprazole, ziprasidone, and quetiapine in the treatment of first episode of psychosis at 3-year follow-up. METHOD:From February 2001 to January 2011, 2 phases of a prospective, randomized, open-label study were undertaken. A total of 376 first-episode drug-naïve patients were randomly assigned to olanzapine (n?=?55), risperidone (n?=?63), haloperidol (n?=?56), aripiprazole (n?=?78), ziprasidone (n?=?62), or quetiapine (n?=?62) and followed up for 3 years. The primary effectiveness measure was all cause of treatment discontinuation. In addition, an analysis based on intention-to-treat principle was conducted in the analysis for clinical efficacy. RESULTS:The overall dropout rate at 3 years reached 20.75%. Treatment discontinuation rates were significantly different among treatment groups (olanzapine?=?69.09, risperidone?=?71.43, aripiprazole?=?73.08%, ziprasidone?=?79.03%, haloperidol?=?89.28%, and quetiapine?=?95.53%) (?2?=?79.86; P?=?.000). Statistically significant differences in terms of lack of efficacy, adherence, and tolerability were observed among treatment groups along the 3-year follow-up, determining significant differences in time to all-cause discontinuation (log-rank?=?92.240; P?=?.000). Significant differences between treatments were found in the categories of sleepiness/sedation, increased sleep duration, akinesia, weight gain, ejaculatory dysfunction, extrapyramidal-symptoms, and amenorrhea. CONCLUSIONS:Olanzapine, risperidone, and aripiprazole presented advantages for the first-line treatment of first episode of psychosis in terms of effectiveness. Identifying different discontinuation patterns may contribute to optimize treatment selection after first episode of psychosis.ClinicalTrials.gov Identifier: NCT02526030 https://clinicaltrials.gov/show/NCT02526030.