Desvenlafaxine vs. Placebo Treatment of Chronic Depression

The investigators are studying a new antidepressant medicine, desvenlafaxine, for the treatment of people with chronic depression. Desvenlafaxine (trade name Pristiq) has been approved by the FDA for the treatment of major depression. The investigators are testing whether this medicine is also effective for adults with a type of chronic depression that is less severe than major depression. This condition is also known as dysthymic disorder or dysthymia. Chronic depression, lasting two or more years, often causes significant suffering and impairment. In addition, the investigators are using MRI imaging, which uses magnetic signals to make pictures of the brain's structure and also of its functioning. The purpose of MRI imaging in this study is to see whether chronic depression is associated with differences in brain structure or functioning, and whether such differences change after medication or placebo treatment. To test this MRI scans are done at the start of the study and after 12 weeks of medication or placebo treatment. Getting MRI imaging will be an option for participants in this study but is not required. This study involves a 6 to 12 week double-blind period during which half of the participants will take the new medication and half will take a placebo (an inactive look-alike pill). After the double blind phase, all subjects can be treated for 12 weeks with an FDA-approved antidepressant medication. Assessments (of depressive symptoms, social functioning, and personality) will be done by study staff and by patients before the study starts, at each study visit for the first 12 weeks, and again after 24 weeks in the study.