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.
In 1998, a prospective study from Erasmus Medical School in the Netherlands, showed that smokers were twice as likely as those who never smoked to develop dementia associated with Alzheimer's Disease. All of the participants started out without symptoms of dementia, and were tested two years later to determine their mental acuity. This study also considered the "gene-environment" factors. It looked separately at people who carried the apolipoprotein E-4 gene that indicates they are more likely to develop Alzheimer's in their lifetime. Surprisingly, these carriers who smoked were not more likely than non-carriers to get dementia.
The same Medical Center published another study in 2004, showing that year to year, the rate of mental decline was significantly worse among those who smoked. In fact, they could even measure a difference between individuals who had smoked in the past, but since quit, and those who had not smoked their entire adult lives. The study was much larger than previous studies, involving almost 10,000 people over 65 years old. There is also increasing evidence that Alzheimer's Disease, as a neurological disorder, might also be considered a vascular disease. There are numerous, indisputable studies that prove smoking is detrimental to vascular health.
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.