No good evidence exists that smoking protects against dementia

Papers p 1097

Smoking prevents dementia? Smoking causes dementia? Over the past decade a succession of research findings has produced apparently conflicting evidence on this question. In the early 1990s results from case-control and family studies suggested a protective effect.1 2 The findings were widely reported,3 including in the mass media, and some scientists stated publicly that they would consider taking up smoking if they had a family history of dementia.

Tobacco companies began to sponsor conferences on dementia, perhaps because it seemed to offer them a lifeline in an otherwise relentless sequence of findings about the deleterious effects of smoking. If smoking reduced life expectancy and also reduced the likelihood of survivors developing dementia then, from a policy perspective, there might be a role for the habit in later life.

The potential protective effects have some biological plausibility. Alzheimer's disease affects neurotransmitter systems, particularly the cholinergic system. Nicotine is a cholinergic agonist. The effect of nicotine on cognition, attention, …