A study published in 2021 made an incredible claim: men who regularly used sildenafil (the generic drug that is the active ingredient in Viagra) had a 69% lower risk of Alzheimer’s Disease (AD). Publications and websites around the world began promoting sildenafil as a preventative medication for AD.
Unfortunately, a subsequent study in 2022 found that sildenafil does not have any benefit in preventing Alzheimer’s.
What Went Wrong?
How could two studies come to such different conclusions?
The problem lies in the interpretation of the first study. Researchers examined insurance data, looking for correlations between the use of medications, and the prevalence of medication conditions. They found that men who used sildenafil had a much lower incidence of Alzheimer’s Disease.
However, there is an important rule among data scientists: “correlation does not imply causation.”
In other words, just because two things occur together, it doesn’t mean that one thing causes the other.
In this case, it’s important to remember that men take sildenafil because they have erection dysfunction. Erectile dysfunction can be caused by many underlying conditions. Apparently, it is one of those underlying conditions, and not the sildenafil, that is reducing the risk of AD.
How Do We Know It’s Not the Sildenafil?
Rather than simply performing a statistical analysis of insurance data, the researchers in the second study conducted what is called a cohort study. They examined two groups of men with similar backgrounds and medical conditions. One group was taking sildenafil, and the second group was not. The incidence of Alzheimer’s was the same in both groups.
The researchers also directly examined tissue samples to determine if sildenafil inhibited the formation of Aβ plaques in the brain, and found that it did not.
What Does This Say About the First Study?
In hindsight, it’s easy to criticize the authors of the initial study, but also very unfair. The authors were certainly aware of the limitations of their initial approach. The news media, however, widely proclaimed results of medical studies they did not understand.
The initial study was part of an ongoing effort to use data analysis to find unexpected connections between medications and medical conditions. The goal of this program is to identify candidates for further research. In this case, subsequent research found that sildenafil is not a preventative for Alzheimer’s Disease. But the same technique may, in the future, find many useful treatments for serious diseases.