This is a 12-week clinical trial evaluating the efficacy and safety of memantine hydrochloride (Namenda) in the treatment of executive function deficits (EFDs) in adults with Attention Deficit Hyperactivity Disorder (ADHD) receiving open-label treatment with OROS-Methylphenidate (OROS-MPH, Concerta). The study aims to examine the effects of treatment with memantine on ADHD symptoms. Following screening procedures, memantine is prescribed in randomized, double-blind fashion (equal chance of medication or placebo) for 12 weeks, along with open-label OROS-MPH (everyone receives medication).