A commonly prescribed drug could help prevent dementia, according to a new study. The findings suggest that Viagra could lower the risk of Alzheimer’s disease.

It was one of three existing medications found to have this potential, alongside the shingles vaccine (Zostavax) and a treatment for motor neurone disease (riluzole). It is hoped these drugs could be repurposed to treat or prevent Alzheimer's disease.

Developing entirely new medications can require 10 to 15 years and cost billions of pounds - with no certainty of success. As reported by Science Daily, repurposing drugs that are already licensed and in widespread use could offer a quicker, safer, and more cost-effective route towards novel Alzheimer's therapies.

As part of the research, which was published in Alzheimer's Research and Therapy, an international group of 21 dementia specialists from universities, hospitals, and the pharmaceutical industry, along with people affected by dementia, evaluated 80 existing medications. Their mission was to identify which ones showed the greatest promise for treating or preventing Alzheimer's disease, which accounts for more than half of all dementia diagnoses.

Following multiple rounds of review, the panel agreed on three “priority candidates” for further research. Each drug was selected because it targets biological processes linked to Alzheimer's, has shown encouraging results in cell and animal studies, and is considered safe for use in older adults.

This was found to have the most promising potential for preventing Alzheimer’s. Immune system changes are known to play a role in Alzheimer's, and this vaccine interacts with the immune system in a way that may help counter some of those harmful changes.

Research suggests it may protect nerve cells and reduce the buildup of tau, a protein that accumulates abnormally in Alzheimer's. In studies involving mice, sildenafil also improved thinking and memory, possibly by increasing blood flow to the brain.

Globally, around 16 million people are thought to use Viagra. In England, nearly 3.5 million prescriptions for Viagra were dispensed between September 2023 and August 2024.

This is currently prescribed for motor neurone disease. Riluzole has improved cognitive performance and lowered tau (a protein linked to Alzheimer’s disease) levels in animal studies.

Experts are now calling for clinical trials to ascertain whether these medications genuinely benefit individuals with Alzheimer's or those at risk of developing the condition. Of the three, the shingles vaccine was particularly notable.

It requires no more than two doses and has a long-standing safety record. Prior research indicates that individuals who received the vaccine were approximately 16 per cent less likely to develop dementia.

Researchers aim to initiate a large-scale UK clinical trial of the shingles vaccine, utilising the PROTECT online registry to monitor participants. PROTECT is an online platform where volunteers fill out annual health and lifestyle questionnaires and participate in brain health research.

Dr Anne Corbett, professor of Dementia Research at the University of Exeter, said: "Beating dementia will take every avenue of research -- from using what we already know, to discovering new drugs to treat and prevent the condition.

"Drug repurposing is a vital part of that mix, helping us turn today's medicine for one condition, into tomorrow's treatment for another.

"It's important to stress that these drugs need further investigation before we will know whether they can be used to treat or prevent Alzheimer's. We now need to see robust clinical trials to understand their true value and know for certain if they are effective to treat or prevent Alzheimer's."

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Prof Fiona Carragher, chief policy and research officer at alzheimer's Society, said: "Dementia devastates lives, but we believe research will beat it. Years ago, we saw aspirin being repurposed from being a painkiller to helping people reduce their risk of heart attack or stroke.

“This is what we want to see in the field of dementia, and why we believe drug repurposing is one of the most exciting frontiers in dementia research."

The study was funded by Alzheimer's Society and led by the University of Exeter, with support from the National Institute for Health and Care Research (NIHR), the Exeter Biomedical Research Centre, and the NIHR HealthTech Research Centre in Brain Health.