The All-Party Parliamentary Group on Dementia (APPG), back in 2008, asserted that perhaps half of dementia patients may be receiving inappropriate medications. While the extent to which this position may have changed in recent years is debatable, the allegation has been picked-up in a recent news item on potentially inappropriate prescribing for the elderly.
Half of Dementia Patients Get Inappropriate Meds. [Online]: Newsmax; Health, October 6th 2017.
This relates to an academic article dating back a few years:
Renom-Guiteras A. Meyer, G. [and] Thürmann, PA. (2015). The EU(7)-PIM list: a list of potentially inappropriate medications for older people consented by experts from seven European countries. European Journal of Clinical Pharmacology. July 2015; 71(7): 861-75.