Smoking has no benefits. Cigarette smoke contains over 4,800 chemicals, 69 of which are known to cause cancer.
A few major studies have established a correlation between smoking and developing Alzheimer's Disease, but none are widespread or detailed enough to fully understand the link. Some early studies were retrospective, meaning they examined the smoking habits of those who had already developed Alzheimer's. Prospective studies followed smokers and non-smokers, administered tests, and measured mental acuity. As it stands, the decline in mental skills of the elderly is worse among smokers. However, in people who carry a gene that makes them susceptible to developing Alzheimer's, smoking seems to neither prevent nor speed the onset of the disease.
One complication in these studies is the tendency of smokers to die earlier than non-smokers from stroke, cancer, or heart disease. Thus, the studies are skewed toward those relatively healthy smokers that have not suffered serious health problems. Also, these studies rely on people's own reporting about their smoking habits, rather than collecting independent verification. Lastly, it has been shown that nicotine, when injected and not inhaled, can improve mental faculties, such as memory recall, of Alzheimer's patients. Certainly, further studies are needed to fully understand the causal relationship between Alzheimer's Disease and smoking.
The researchers also looked into how smoking affects the risk of developing Alzheimer's disease for people who have the gene that increases the risk of Alzheimer's, called apolipoprotein E4, or APOEå4. They found that smoking did not increase the risk of Alzheimer's for those with the APOEå4 gene. But for those without the APOEå4 gene, smoking increased the risk of Alzheimer's. Current smokers without the Alzheimer's gene were nearly 70 percent more likely to develop Alzheimer's than nonsmokers or past smokers without the Alzheimer's gene.