Antibiotics, antivirals and vaccines are associated with a lower risk of dementia, a study has found, providing more evidence for the theory that infections could trigger the disease.
Researchers at the universities of Cambridge and Exeter said anti-inflammatory drugs were also associated with a lower odds of developing the condition.
The discoveries suggest several already licensed drugs could be repurposed to treat dementia.
The latest research looked at health data from more than 130 million people, including one million dementia cases, from 14 studies. Although they found a lack of consistency between studies in identifying those drugs that could affect the risk of dementia, they identified several potentially useful medicines.
One unexpected finding was the association between antibiotics, antivirals and vaccines, supporting recent interest in the BCG vaccine for tuberculosis, for example, which previous studies have found can defer dementia onset with an efficacy better than all drugs known to date.
Anti-inflammatory drugs, which were also found to be linked to reduced risk, are increasingly being seen as a significant contributor to tackling a wide range of diseases. That could be because some genes that increase the risk of dementia are part of inflammatory pathways, said the researchers.
Dr Ben Underwood, who jointly led the research, told The National that the study did not explain why the association exists.
"It’s possible they may have some influence on the dementia process, it's possible they might reduce risk through their antimicrobial action or it is possible that they may not be causally linked, for example people who get vaccinated may be more health conscious, less likely to smoke and this explains the lower risk of dementia," he said. "It is also true to say that no drug appeared consistently in our study."
For other drugs, the evidence was less clear cut. Some blood pressure medications and anti-depressants and, to a lesser extent, diabetes medication were associated with a decreased risk of dementia. Others were associated with a raised risk.
“Because a particular drug is associated with an altered risk of dementia, it doesn’t necessarily mean that it causes or indeed helps in dementia,” said Dr Ilianna Lourida of the University of Exeter.
“We know that diabetes increases your risk of dementia, for example, so anyone on medication to manage their glucose levels would naturally also be at a higher risk of dementia – but that doesn’t mean the drug increases your risk.”
Researchers are yet to find drugs that are effective in slowing or preventing dementia, despite intensive efforts. Until recently, medicine developed to treat the condition have proven to have only a modest effect on symptoms.
Lecanemab and donanemab have been shown to reduce the build-up in the brain of amyloid plaques, a key characteristic of Alzheimer’s, and to slow progression of the disease. But the National Institute for Health and Care Excellence concluded the benefits were insufficient to justify approval for use within the NHS in Britain.
So scientists are increasingly turning to existing drugs to see if they may be repurposed to treat dementia. As the safety profile of these drugs is already known, the move to clinical trials can be sped up significantly.
“We urgently need new treatments to slow the progress of dementia, if not to prevent it,” said Dr Underwood.
"If we can find drugs that are already licensed for other conditions, then we can get them into trials and, crucially, may be able to make them available to patients much, much faster than we could do for an entirely new drug. The fact they are already available is likely to reduce cost and therefore make them more likely to be approved for use in the NHS.”
However, he told The National that more would need to be known about the association and how it might arise before they are approved for use in the treatment or prevention of dementia. Large-scale trials are planned in the UK to test existing drugs for dementia treatment, so these medicines could be the focus of studies this year.
"If shown to be beneficial because they are already licensed, they could be rapidly available to patients," he said.
The study, published in Alzheimer’s and Dementia: Translational Research & Clinical Interventions, comes after other research recently found that head injuries may waken viruses “sleeping” in the brain, causing damage that could lead to neurodegenerative diseases such as dementia, Alzheimer’s and Parkinson’s.
The relationship between neurodegenerative diseases and repeat concussion and head injuries in sports such football and boxing is already well known, but researchers do not yet fully understand why it happens.
A new study may help explain the link, suggesting head knocks can awaken latent viruses in the nervous system, setting off a chain of inflammation that results in damage over time. Suspected culprits include herpes simplex virus 1 (HSV-1), which is found in more than 80 per cent of people, and varicella-zoster virus, found in 95 per cent of people and which make their way into the brain, and sleep within its neurons and glial cells.
Previous research has found that reactivation of HSV-1 causes Alzheimer’s symptoms in lab models of brain tissue.
Researchers say antiviral drugs could be used to help prevent post-head injury to guard against future illness.
