Weight gain is an adverse effect of antidepressants and antipsychotics. This side effect can lead to numerous comorbidities and reduces life expectancy. The use of these drugs is increasing worldwide, and the weight gain produced by them represents a common clinical challenge. The goal of this systematic review was to evaluate the potential association of antidepressant and antipsychotic therapy with body weight gain in cohort studies. A search of cohort studies investigating the association between weight gain and the use of antidepressants and antipsychotics in individuals was conducted through the PubMed database from 1 January 2008 to 31 January 2019 following the PRISMA statement. We found 27 independent eligible cohort studies that included children (2-18 years old) and adult (18-103 years old) subjects.Most of the included studies showed a 5% weight gain in individuals using antidepressant therapy. However, Quetiapine, Haloperidol, Trifluoperazine, Risperidone, Aripiprazole, Olanzapine, and Clozapine increased body weight ≥7% from baseline, which is considered a clinically significant result. Weight loss was found in individuals treated with Bupropion. Further cohort studies with higher sample sizes and longer durations of treatment are needed to confirm our observations. KEYWORDS antidepressant, antipsychotic, drug, weight gain, mental disorders 1 | BACKGROUND During the last decade, the prevalence of mental disorders has increased, comprising of 22.8% the global disease burden 1 and causing significant effects on people's health with negative socio-economic consequences. 2 There are 350 million people affected by mental disorders around the world 3 . The disorders include depression, bipolar disorders, schizophrenia and other psychosis, dementia, and developmental disorders (autism spectrum disorder [ASD]). The World Health Organization (WHO) indicates that mental disorders affect people from any age, social condition, or country. 4 Depression is the most common mental disorder. This disease may produce mental distress and also influences simple daily tasks affecting family and work relationships. 2 Moreover, depression is considered as the main cause of disability, contributing to a high morbidity and mortality. 2 In the worst-case scenario, depression can lead to suicide, which nowadays is considered the second leading cause of death among people 15 to 29 years old. 4 When talking about this disease, the WHO gives special attention to the three most affected population groups nowadays, which are young people from 15 to 24, women in childbearing age (especially after child birth), and people over 60 years old. The WHO estimates that the number of people affected