Alzheimer's disease (AD) is the most common cause of dementia in adults 65 years and older. AD leads to a complete loss of memory and independent function, and presently there is no cure. Many studies suggest that lithium treatment may delay dementia onset or slow its progression. However, more research is needed to understand the extent of its anti-dementia properties if it will be deployed broadly in the general population. This study will examine whether lithium has anti-dementia properties in older adults who have mild cognitive impairment and are at risk of becoming demented.