Memantine and Down's Syndrome

This is a study to assess whether memantine is effective and safe in preventing age related cognitive deterioration and dementia in people with Down's syndrome (DS) age 40 and over. The study will last for a year and it will include 180 people with Down's syndrome with and without dementia. Participants will be assessed on memory skills, attention and problem solving abilities. Quality of life and abilities for everyday living skills will also be regularly checked. Primary Aims Clinical: * To determine the clinical efficacy of memantine versus placebo in preventing cognitive decline in people with DS. * To compare the safety and tolerability of memantine versus placebo in people with Down's syndrome (DS). Biochemical and pathological: * To examine the ability of memantine to alter markers of disease progression in DS patients. Secondary Aims Clinical: * To determine whether memantine has, as compared with placebo, a significant positive impact on: * level of independent functioning as measured by the carer-rated adaptive behavioural scale, (ABS) in adults with DS; * quality of life in adults with DS. Biochemical and pathological: * To investigate putative markers of memantine's mechanism of action in peripheral samples from living patients with DS.