Aricept (and Reminyl and Exelon) act by increasing the amount of a chemical called a neurotransmitter in the brain. This chemical is involved in many thought processes. People with Alzheimer's have a shortage of this chemical. So the drugs work a bit like insulin. They do not treat the actual disease, and they do not slow down it's progress as such. However, they can reduce the symptoms of the disease. This means that in practical terms the progress is slowed because people taking the drug take longer to decline than people who don't. However, the reaction to the drug is very individual, some people get absolutely no benefit from it, some people may even get worse. It is quite difficult to measure the success, because it may only be possible to say there is a benefit from it if you stop the drug and they get worse. The usual measure is to do regular checks using the MMSE exam, and to compare the rate of decline with the rate that would be expected without the drug. My father definitely showed signs of improvement when he started taking it, and although the dementia has progressed as we expected, the drug meant that he had an "improves startign point" (if that makes sense), so effectively, his decline will take longer than it would have otherwise. I would judge that we "gained", roughly speaking, about 2 1/2 years.