Sildenafil and shingles vaccine show surprising promise against Alzheimer’s, suggests study

Researchers have pinpointed three already approved medications that may be repurposed to treat or prevent Alzheimer's disease. Instead of starting from scratch, scientists examined medicines that are currently used for other conditions to see whether any could help protect the brain.

The study, funded by Alzheimer's Society and led by the University of Exeter, was published in Alzheimer's Research and Therapy. Among the drugs reviewed, a shingles vaccine (Zostavax) emerged as the most promising candidate. Viagra (sildenafil) and a medication used to treat motor neurone disease (riluzole) also showed strong potential.

Why Drug Repurposing Matters

Dementia is the leading cause of death in the UK, affecting around one million people. One in three people born today will develop dementia in their lifetime -- yet there is still no cure.

Creating brand new drugs can take 10 to 15 years and cost billions of pounds -- with no guarantee of success. Repurposing medicines that are already approved and widely used could provide a faster, safer, and more affordable path toward new Alzheimer's treatments.

This work was also supported by the National Institute for Health and Care Research (NIHR), the Exeter Biomedical Research Centre, and the NIHR HealthTech Research Centre in Brain Health.

How the Top Alzheimer's Candidates Were Chosen

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 goal 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.

After 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.

The three priority drugs are:

• Shingles vaccine (Zostavax)- Studies suggest a possible connection between the shingles virus and dementia. 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.

• Sildenafil (Viagra) -- Research indicates it may protect nerve cells and reduce the buildup of tau, a protein that accumulates abnormally in Alzheimer's. In mouse studies, sildenafil also improved thinking and memory, possibly by increasing blood flow to the brain.

• Riluzole -- Currently prescribed for motor neurone disease, riluzole has improved cognitive performance and lowered tau levels in animal studies.

Shingles Vaccine Shows Strongest Signal

Experts are now calling for clinical trials to determine whether these medications truly benefit people who have Alzheimer's or are at risk of developing it.

Among the three, the shingles vaccine stood out. It requires no more than two doses and has a long record of safety. Previous research suggests that people who received the vaccine were about 16% less likely to develop dementia.

Researchers hope to launch a large UK clinical trial of the shingles vaccine, using the to track participants. PROTECT is an online registry in which volunteers complete annual questionnaires about their health and lifestyle and take part in brain health research.

Other Drugs Considered

Five additional medications were shortlisted but did not meet the criteria to be named 'priority candidates.' These included fingolimod (used in MS), vortioxetine (used to treat major depressive disorder), microlithium (used to treat depression), dasitinib (used for leukaemia), and cytisine (used in anesthetics).

Experts Urge Caution and Further Trials

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."

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."

Reference:

Anne Corbett, Janet Sultana, Kate Stych, Roger Mills, Jeff L. Cummings, Gareth Williams, Zahinoor Ismail, Maria Soto-Martin, Jacobo Mintzer, Serge Gauthier, Nigel H. Greig, Wendy Noble, Richard Killick, Mitchell K. P. Lai, Carol Routledge, Frank Walsh, Howard Fillit, Dag Aarsland, Roger Lane, Kathryn Mills, Clive Ballard. Drug repurposing for Alzheimer’s disease: a Delphi consensus and stakeholder consultation. Alzheimer-'s Research, 2025; 17 (1) DOI: 10.1186/s13195-025-01895-4