The population of dementia is increasing rapidly. Cognitive impairment as well as Behavioral and psychological symptoms of dementia (BPSD) add heavy burdens to caregiver. NMDA activation is critical for learning and memory. Individuals with Alzheimer's disease (AD) have fewer NMDA receptors in the frontal cortex and hippocampus than controls. This study is a randomized, double-blind, placebo-controlled drug trial. All patients will be allocated randomly to two groups: (1) NMDA enhancer: DAOIB (starting dosage: 250-500 mg/day), (2) placebo, for 6 weeks. The investigators hypothesize that DAOIB may yield better efficacy than placebo for cognitive function and clinical symptoms in patients with BPSD.

Inclusion Criteria:

• Clinical diagnosis of probable Alzheimer's disease or probable vascular dementia
• For patients with vascular dementia, the post-stroke period must be more than 3 months
• Mini-Mental State scores between 5-26
• Clinical Dementia Rating score equal to or greater than 1
• Behavioral Pathology in Alzheimer's Disease Rating Scale score equal to or greater than 2

Exclusion Criteria:

• Current substance abuse or history of substance dependence in the past 6 months
• Other major psychiatric diagnoses, such as schizophrenia, major depressive disorder, bipolar disorder and mental retardation etc.
• Serious medical or neurological illness other than Alzheimer's disease/vascular dementia and other secondary dementia