Alzheimer's disease (AD) is multi-factorial mental disorder characterized by a copious array of congruent features cumulating in disrupted memory and dysthymia. Though the mechanism remains elusive, the highly unspecific pharmaceutical, memantine, provides modest benefits for patients with moderate-to-severe AD. A greater understanding of how memantine affects cognitive function promises to facilitate the search for better therapeutics. We therefore examined cognitive flexibility of mice following 5 and 10 mg/kg memantine administration using a platform re-location water maze. Strikingly, subjects receiving memantine demonstrated memory impairment relative to controls when re-trained off drug, revealing a novel and unusual disruption of cognitive flexibility.