Open label memantine will be titrated in 5mg increments weekly to target dose of 10mg po bid for up to 6 weeks. Memantine will be continued to 12 weeks in those with treatment response,13 either previous response to ketamine (≥ 35% Y-BOCS reduction 1 week after IV ketamine) or current response to memantine (≥ 35% Y-BOCS reduction from pre- to post-6 weeks of memantine).