Smoking has little net effect on the incidence of severe dementia

The epidemiological evidence that smoking might offer some protection against Alzheimer's disease has come from small retrospective studies of uncertain reliability. A large prospective study of smoking has been in progress for almost 50 years among British doctors, recording their smoking habits every few years and monitoring their death certificates. As the doctors grow older, an increasing proportion who die have dementia severe enough to be mentioned on the death certificate. On p 1097 Doll et al report that the likelihood of dementia being mentioned is similar for continuing smokers and long term non-smokers, for both dementia as a whole and dementia attributed to Alzheimer's disease.