Doctors are to launch an investigation into a group of drugs used to treat high blood pressure, after a study found they may cut the risk of Alzheimer's disease and other dementias.
People taking the drugs, known as angiotensin receptor blockers, or ARBs, had a 35-40% lower risk of developing Alzheimer's disease and similar neurodegenerative disorders, according to medical records of more than 5 million patients.
Researchers at Boston University school of medicine said the drugs also appeared to slow the progression of Alzheimer's disease, reducing deaths, admissions to nursing homes and certain symptoms of the condition by up to 45%.
The findings are so striking that doctors heading the study have been given immediate funds to investigate the medical records of a further 3 million patients.
If the drugs' effects are confirmed, they could potentially transform the treatment of a disease that leads to serious debilitation and sometimes death in 15- 20 million people worldwide, and which has been estimated to cost around $250bn (£125bn) a year in treatment and care.
Benjamin Wolozin, a professor of pharmacology, used a huge database of medical records held by the US department of health to compare the health of patients who took ARBs with those who took other drugs.
"When we took account of the effects of age and cardiovascular disease, strokes and diabetes, or anything else that could influence a patient's outcome, these medications still looked very impressive. Potentially, this will have a large public health impact," Wolozin said.
The drugs were of most benefit to patients who had experienced a stroke.
Of the 700,000 people in Britain who have dementia, more than half have Alzheimer's.
As the population ages, that number is expected to rise to nearly 1 million in less than 20 years.
Last year, doctors in Britain wrote more than 13m prescriptions for ARBs for the treatment of high blood pressure. The most common ARBs are marketed as candesartan, losartan and irbesartan.
Wolozin, who is due to announce details of the study at the international conference on Alzheimer's disease in Chicago today, is uncertain precisely how the drugs protect against dementia.
Any drug that reduces high blood pressure should lower the risk of dementia to some extent, but ARBs seem to have a much larger benefit than expected.
It is possible that ARBs help prevent dementia by protecting nerve cells in the brain.
Experiments on animals suggest ARBs shield brain cells from damage and help them recover from injuries caused by ageing and faulty blood vessels.
"This works on two levels. On the public health level, this could reduce entry to nursing homes by so many millions and save however many millions of dollars.
"But on the personal level, if your relative is taking this you might not see any difference, day to day, but the hope is that rather than going into the nursing home next year, they go in two or three years' time instead. It's not like it's a slam-dunk cure," said Wolozin.
In the next few months, Wolozin's team will repeat the study using medical records of a further 3 million patients held by US healthcare giants, Kaiser Permanente.
If that confirms the effect of ARBs on dementia, it will pave the way for longer clinical trials, although many doctors would probably not wait for trials to finish before prescribing the drugs to Alzheimer's patients.
Wolozin said his team has identified other drugs that appear to prevent Alzheimer's and some that make the condition worse.
By combining ARBs with other drugs, he believes it may be possible to reduce the risk of dementia by more than 50%.