Deep brain stimulation (DBS) is increasingly investigated as a therapy for psychiatric disorders. In the ethical evaluation of this novel approach, incidence and impact of side effects (SE) play a key role. In our contribution, we analyze the discussion on SE of DBS of the subthalamic nucleus (STN)—a standard therapy for movement disorders like Parkinson's disease (PD)—based on 66 case reports, 69 review papers, and 347 outcome studies from 1993 to 2009. We show how the DBS community increasingly acknowledged the complexity of STN-DBS side effects. Then we discuss the issue of study quality and the methods used to assess SE. We note that some side effects are the subject of conflicting evaluations by the different stakeholders involved. This complicates the ethical controversy inherent in any novel treatments for diseases that involve psychiatric aspects. We delineate how the lessons from STN-DBS could guide future DBS applications in the field of psychiatry.