Bariatric surgery is considered an effective treatment against obesity. This intervention and the subsequent weight loss can affect several pharmacokinetic parameters leading to a possible need of dosing adjustment. Despite an increasing number of bariatric interventions, clinical recommendations are often based on theoretical principles and expert opinions. Because reliable clinical and biological parameters for adjusting psychotropic drug are not available, the use of therapeutic drug monitoring needs to be advised as a useful approach. However, it is not currently possible to predict the effect of bariatric surgery on psychotropic blood levels, and due to their important inter-individual variability, preoperative medication blood levels should be measured to ensure an adequate postoperative dosing adaptation.