At one time statins were considered to be a possible treatment for Alzheimer's disease.  But this small study suggests that statins may actually increase cognitive decline in Alzheimer's patients. 

http://www.ncbi.nlm.nih.gov/pubmed/22921881 

http://www.drbriffa.com/2013/01/31/statins-worsen-brain-function-of-alzheimers-sufferers/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+drbriffa%2FsOZf+(Dr+Briffa's+Blog+-+A+Good+Look+at+Good+Health)

Statins lower Coenzyme Q10 levels, and Coenzyme Q10 as an antioxidant may have some limited benefit in slowing down Alzheimer's disease.  

http://www.mayoclinic.com/health/coenzyme-q10/NS_patient-coenzymeq10/DSECTION=evidence 

Statins have been linked to global amnesia, but not yet to an increased risk for Alzheimer's disease (it has properties that both increase and decrease the risk for Alzheimer's disease).  Once people stopped taking statins the amnesia seemed to go away.   

Nevertheless, at some point there should be a thorough review of what drugs increase the risk for Alzheimer's disease (one preliminary study found about 130 drugs that impaired cognitive function) and which drugs 

speed cognitive decline in people with Alzheimer's disease.