Adult outpatients with major depressive disorder and partial response or nonresponse to their current antidepressant ( as indicated by a 17-item Hamilton Depression Rating Scale score of 16 at baseline ) were randomized ( from July 2006-December 2011 ) to add memantine ( flexible dose 5-20 mg/d , with all memantine group participants reaching the dose of 20 mg/d ) ( n = 15 ) or placebo ( n = 16 ) to their existing treatment for 8 weeks .