More than 1600 drugs were studied and researchers looked at how they interacted with more than 350,000 different proteins in the human body.
This method is a new field of investigation and could lead to medicines being repurposed to expand their range of applications. Analysis revealed that sildenafil may be effective at targeting some proteins involved in the development of Alzheimer's in the brain.
Alzheimer's is the most common manifestation of dementia, and is one of the UK's biggest killers. In 2020, 11.5 per cent of all registered deaths in England and Wales were attributed to Alzheimer's or dementia, with only Covid accounting for a higher proportion of deaths.
Trawling through insurance records
Previous studies on animals have indicated that Alzheimer's may be preventable and treatable with Viagra, but there has been no hard evidence from human trials.
In order to firm up the evidence base, the academics trawled through the medical insurance records of more than seven million people in the US.
This anonymised dataset allowed the researchers to look at those who were prescribed the drug and their health outcomes over several years.
It showed that about 99 per cent of people taking Viagra did not develop Alzheimer's over the six-year span. For those not on the drug, the figure was about 95 per cent.
"After six years of follow-up, sildenafil usage was significantly associated with a 69 per cent reduced risk of Alzheimer's disease, compared with matched non-sildenafil users," the scientists wrote in their paper, published in the journal Nature Aging.
Nearly all people, 98 per cent, in the study that were taking Viagra were male as a consequence of the drug's current use and the researchers failed to find hard evidence that the drug lowered the risk of Alzheimer's in women.
The study has demonstrated a link between those who take the drug and a lower risk of Alzheimer's disease, but the data is unable to prove Viagra is the reason for this.
Experts caution that the only way to ascertain a demonstrable causation is with a clinical trial.
Opinion split over effectiveness
Robert Howard, the professor of old age psychiatry at University College London, said that the results "failed to excite" him.
"Association does not imply causation and one only has to pause for a moment to consider the characteristics of older men who consult with their doctor for drug treatment of erectile dysfunction to understand why they might already be different from someone in the pre-symptomatic stages of Alzheimer's disease," he added.
However, some academics were more upbeat about the research. Dr Jack Auty from the University of Tasmania, for example, said there were clear issues with the paper, but the approach and findings were "exciting", "fascinating" and "fantastic".
"We need further research. In the field of Alzheimer's disease research, we have been excited by many drugs over the years, only to have our hopes dashed in clinical trials. I will be following this research group and the research around sildenafil closely."
Dr Susan Kohlhaas, the director of research at Alzheimer's Research UK, said: "Developing drugs for diseases like Alzheimer's, which attack the brain, is a costly process and can take many years.
"Being able to repurpose a drug already licensed for other health conditions could help speed up the drug discovery process and bring about life-changing dementia treatments sooner."
Dr Ivan Koychev, the senior clinical researcher at the University of Oxford, called the study "an exciting development", adding: "This 'repurposing' approach can reduce the drug discovery period by years and reduce the risk of failure as we already know how safe these drugs are in humans."
