Hyperprolactinemia is one of the commonest side effects in patients taking antipsychotics, and the tendency of antipsychotics to elevate plasma prolactin level is dose dependent. This study aims to investigate the relationship between use of antipsychotic medications and hyperprolactinemia among mentally ill patients. This is a longitudinal study. From 90 consenting antipsychotic naïve patients and all of whom have met the inclusion criteria, sociodemographic interview schedule was administered. Next, venous blood sample was collected for estimation of serum prolactin level and repeated after 8 weeks. Overall prevalence of hyperprolactinemia is 30%; there is a correlation between hyperprolactinemia and chlorpromazine equivalent dose of antipsychotics (r = 0.397; p < 0.001); there is no association between hyperprolactinemia and class of antipsychotic, and the greatest predictor of hyperprolactinemia was found to be the dose of antipsychoti c medication used. Hyperprolactinemia may result from the use of typical or atypical antipsychotic medications, and the dose of antipsychotic medication used is the greatest predictor of hyperprolactinemia.