These two examples are from the Alzheimer's Society website:
What are the benefits of cholinesterase inhibitors?
In clinical trials of all three cholinesterase inhibitors, people taking the medications performed better on memory and thinking tests than those taking a placebo, or inactive substance. The degree of benefit was small, however, and more than half of the recipients showed no improvement at all.
What are the benefits of memantine?
One clinical study showed that people taking memantine showed a small but statistically significant improvement in their mental function and ability to perform daily activities. But study participants with lowest cognitive functioning showed no improvement on either daily activities or overall function.
This kind of thing makes it all the more puzzling that there's a big campaign to encourage people to come forward and collect their diagnosis so that they can be 'helped' - presumably with these and equally pathetic drugs.