"There is some evidence that exposure to nicotine, which is one of the components of cigarette smoke, can actually reduce the risk of dementia. Such reports may be useful in indicating possible research directions for drug design.

However, nicotine intake through smoking would not be beneficial. Any positive effects would be outweighed by the significant harm caused by the other toxic components in cigarette smoke." https://www.alzheimers.org.uk/about-dementia/managing-the-ri...