The world health organization (WHO) has defined adverse drug reaction (ADR) as ’A response to a drug which is noxious and unintended and which occurs at doses normally used in man for the prophylaxis, diagnosis, or therapy of disease, or for the modifications of physiological function’. Hospitals are required to evaluate, monitor, and respond to adverse drug reactions (ADRs) in order to obtain accreditation by the joint commissions. Neurotic disorders, psychosis, alcohol/drug addiction, mental retardation and epilepsy are common mental and behavioral disorder in India. Most mental illness require medications for longer duration ranging from several months to years. Because of longer duration of therapy wide range of adverse drug reactions (ADRs) are associated which can lead to long compliments and, or discontinuation of therapy. The wide variation in the clinical response to a standard dose of an antidepressant drug is due to inter and intra individual difference in pharmacokinetics and pharmacodynamics. Under the same dose, some patients respond, others suffer from the poor efficacy or experience toxic effects. The purpose of this study is to employ a mechanistic approach to prospectively investigate the incidence of ADRs in psychiatric inpatients population.