I find that loss of insight varies a lot from one person with Alzheimer's disease to the next. Some people, whose dementia otherwise is quite typical, will lose insight early in the course of their illness, whereas for others it is quite preserved. Very occasionally - I have seen it in about one patient in a thousand, but I have heard about it more often - some people with Alzheimer's disease who are treated will regain insight into their impairments, without regaining much function. This is a "worst case scenario" in which they have no beneficial treatment effect, and the only effect they have is distressing. Although very uncommon in my experience, I have often heard this story told by people who are against drug treatment of Alzheimer's disease.