Participants will enter a 2-week, single-blind, placebo lead-in phase, during which they will visit the clinic three times each week. At each study visit, urine samples and other rating assessments will be collected. In addition, participants will attend weekly therapy sessions. In order to continue in the trial, participants are required to attend at least four out of the first six study visits and both therapy sessions. Eligible participants will then be randomly assigned to receive either memantine or placebo for the duration of the 12-week, double-blind phase of the trial. Study visits will continue to occur three times each week; participants will also receive weekly therapy. Memantine will be taken twice each day. Participants who complete the 12-week trial will enter a 2-week lead-out phase, during which they will be tapered back to a placebo in a single-blind manner. Weekly psychotherapy sessions will continue until the end of Week 14.