The aim of this study is to investigate whether sodium benzoate is superior to placebo in decreasing symptoms among patients with attenuated/transient psychosis. A total of 140 patients will be randomized in 1:1 ratio to receive sodium benzoate 1 g/day or placebo for 12 weeks. Concerning statistical power, the number of patients is sufficient to obtain statistical significance for a clinically meaningful effect size of 0.40 (Cohen's d). The primary outcome measure is change in PANSS sum score of delusions, hallucinations, suspiciousness and conceptual disorganization (the PANSS items that are inclusion criteria) at week 12. Change in CGI score at week 12 is the other primary outcome measure. The secondary outcome measures are change in PANSS total score at week 12, CGI score at week 24, and GAF at weeks 12 and 24.