Drugs commonly used to treat patients at risk from heart disease could help stave off dementia as well.

Statins reduce the amount of cholesterol in the bloodstream, thereby helping keeping vital blood vessels unclogged.

Normally this is a method of protecting middle-aged and elderly patients from a potential heart attack.

But doctors are wondering whether there might be other benefits to keep the blood supply to the brain healthy.

Doctors have long suspected a deterioration of the blood supply to the brain over the years is contributory to some forms of dementia, even Alzheimer's disease.

Some studies suggest long-term smoking - known to be bad for your circulation - is linked with a higher risk of developing Alzheimer's disease.

The latest study took information from more than 350 GP practices and looked for those patients with diagnosed dementia, comparing them to more than 1000 ordinary patients.

Significantly lowered risk

They found patients aged 50 or above given statins had a substantially lowered risk of developing any sort of dementia, including Alzheimer's disease.

One in ten people over the age of 65 develop some form of dementia.

In many cases, it is a progressively worsening condition, leading to memory loss, confusion and in some cases death.

There are few active treatments for Alzheimer's disease. Some drugs appear to be able to reduce the symptoms slightly in some patients.

Dr Richard Harvey, director of research for the Alzheimer's Society, said: "This is new evidence but it is important to note that it is an epidemiological study rather than a randomised controlled trial.

"It specifically looked at people who have a high blood fat level and have been prescribed statins to lower cholesterol.

"This means that it can't be seen as conclusive evidence of a link between statins and a reduced risk of developing dementia, as people taking statins may have other things in common which could be related to risk of developing dementia.

"That said, it is certainly very important evidence. This re-enforces current thinking about possible links between Alzheimer's disease and vascular health."