Dementia is a broad category of brain diseases that cause long term loss of the ability to think and reason clearly that is severe enough to affect a person's daily functioning. For the diagnosis to be present it must be a change from how the person was previously. The most common form of dementia is Alzheimer's disease (75%). Except for a few treatable types in most cases there is no cure. Cholinesterase inhibitors are often used early in the disease course; however, benefit appears to be slight.