The study consists of three phases. In Phase 1, participants will be tapered off all psychiatric medications over a 2-week washout period. In Phase 2, participants will be randomly assigned to receive either memantine or placebo (an inactive pill) three times a day for 8 weeks. Participants who do not respond to the treatment after 8 weeks will be taken off the study and offered standard treatment. Weekly psychiatric evaluations will evaluate treatment response. During Phase 2, participants who respond well to treatment will enter Phase 3, a 16-week continuation phase of either memantine or placebo. Interviews will be conducted every other week in the first month , then monthly thereafter.