Donepezil and Memantine in Moderate to Severe Alzheimer's Disease

This trial will involve the withdrawal of drug participants that are currently on (donepezil) and in arm 4, the participant will only be on placebo treatment. It is important to include this arm of the study as a key objective in looking at the benefit of continuing donepezil and therefore a placebo arm should be present as a comparator. To reduce the risk to participants of withdrawing donepezil too early in their illness, an inclusion criteria is that the participant is at a stage in their disease whereby the prescribing clinician feels a change in drug prescription may be appropriate.