The purpose of this research of 400 participants is to determine whether a drug called risperidone can decrease symptoms of Post-Traumatic Stress Disorder (PTSD). It is a placebo-controlled study, meaning that half of the participants will be assigned to receive a pill that contains no drug. The treatment phase of the study will last for 6 months, during which time participants will continue to receive all their usual treatments in addition to the study treatment and will be asked to complete procedures and assessments (questionnaires, interviews, laboratory tests, physical exams, etc.) related to their PTSD symptoms at various points within the 6-month treatment phase. At the end of the 6-month study, participants will discontinue the study treatment.