A study of outpatient participants with Alzheimer's disease or a related dementia who have difficult behaviors that are upsetting for them or their caregivers. Prazosin is a medication that is commonly used to treat people with high blood pressure. Research with prazosin has shown that it may be effective in treating behavioral problems by reducing excess adrenalin effects in the brain.

This is a 24 week study with 14 visits to the research clinic. Approximately 6 of these visits may be done by phone. Additional phone checks are scheduled at the beginning of each 12 week part of the study. Participants will have a 50:50 chance of being on prazosin or placebo in the first 12 weeks of the study. For the second 12 weeks, all participants will take prazosin.

Study visits include a physical and neurological exam; memory testing; interviews with the caregiver about behaviors; and vital signs.

Inclusion Criteria:

• No age limit
• Probable or Possible Alzheimer's Disease
• Disruptive agitated behaviors at least twice a week (overly anxious or excited, making offensive comments.....)
• Stable medications for 2 weeks
• Must have a caregiver who spends 10 hours per week caring for the participant and agrees to participate in all evaluation sessions

Exclusion Criteria:

• Cardiovascular: unstable angina, recent myocardial infarction, preexisting hypotension (systolic BP less than 110) or orthostatic hypotension (≥20 mmHg drop in systolic BP following 2 minutes of standing posture)
• Any unstable medical condition
• Exclusionary medications: current treatment with prazosin, other alpha-1 blockers (trazodone, sildenafil, vardenafil or tadalafil)
• Psychoactive medications: subjects may be psychoactive medication-free or be partial responders (by subjective assessment of referring health care professional) to one psychoactive medication from any of the following classes: antipsychotics, anticonvulsants, mood stabilizers, antidepressants, benzodiazepines, or buspirone. Partial response is defined as some improvement in agitated behavior but persistence of agitated behaviors severe enough to cause patient distress and/or difficulty with caregiving. Although not formally rated, this improvement is equivalent to a Clinical Global Impression of Change rating of no more than minimal improvement (improvement is noticed by not enough to improve patient function or caregiver's practical management of the patient).
• Psychiatric/behavioral: lifetime schizophrenia; current delirium, mania, depression, or uncontrolled persistent distressing psychotic symptoms (hallucinations, delusions), substance abuse, panic disorder, or any behavior which poses an immediate danger to patient or others or which results in the patient being too uncooperative to meet the requirements of study participation.

12 weeks.

Known, common side effects for prazosin were specifically queried at each study visit.