The study consists of two sequential phases, each lasting a total of four weeks (8 weeks total), with visits every 10 days. Once patients agree to participate in the study by signing the informed consent document, a full medical and psychiatric history will be taken and a physical examination and blood draw will be performed. Screen rating scales will be performed. Screened and eligible patients will be asked to return two weeks later for a baseline visit when they will be randomized to double-blind treatment with placebo or Deplin (6(S)-5-MTHF). The double-blind treatment will last 60 days, during which patients will be seen every 10 days. Subjects will be randomized to one of three treatment groups: a)Deplin/Deplin, b) placebo/Deplin, c) placebo/placebo. Patients will have a greater than 50% chance of receiving active medication (Deplin) at some point in the 8 week study
* For patients randomly assigned to the Deplin/Deplin sequence, the dose of 6(S)-5-MTHF will be 7.5 mg/day during the first phase of the study, and 15 mg/d during the second phase of the study.
* For patients randomly assigned to the placebo/Deplin sequence, the dose of 6(S)-5-MTHF will be 7.5 mg/day during the second phase of the study.
* For patients randomly assigned to the placebo/placebo sequence, both tablets of study medication will be placebo during both phases of the study.
All patients will be asked to take two tablets of blinded study medication in the morning, in addition to their stable dose of ongoing SSRI treatment. Each study medication tablet will be either 7.5 mg of 6(S)-5-MTHF or matching placebo.
