The neurodegeneration of Alzheimer's disease and other dementias result in a loss of brain tissue. As well as the invariable symptom of loss of memory, depression frequently occurs as a secondary symptom. Often presenting relatively early in the condition, symptoms of depression include lowered mood and tearfulness or a change in behaviour. Depression is thought to be more common in non-Alzheimer disease dementias. As a part of a dementia syndrome depression can occur in the early stages, perhaps as a result of retained insight. Even partial knowledge of deterioration and progressive loss of ability can be profoundly distressing.

It has been hypothesized that depression and behavioural disturbance in dementia are due to reduced serotoninergic (serotonine is a hormone of the nervous system) function and has been suggested that SSRIs (selective serotonine re-uptake inhibitors) might be useful in treating some symptoms in dementia.