There is in no way sufficient evidence in this study to draw this conclusion. This study was in no way designed to provide causal evidence. At best, what we can say is that the people who tend to take sildenafil also are less likely to have alzheimers. It could very well be that the people who take sildenafil are more active, and that activity level staves off alzheimers. Or any number of other third variables. This is not as big a deal as the article is making it out to be. Spurious correlations happen all the time, and it behooves us to be measured in our excitement.