A new analysis of 400,000 Americans who took statins during a two-year period found that for many people, the higher their use, the lower their risk of Alzheimer's disease over the following five years.

The research looking at people aged over 65 for seven years discovered there was a 15 per cent lower incidence of the disease among women who had high exposure to statins (people who filled the most prescriptions over two years).

For men in the same category, there was a 12 per cent lower incidence of the disease.

Previous studies have shown a link between cholesterol and the beta-amyloid plaques that interfere with memory and other brain functions.

However, the results varied between ethnic groups and sex, and according to the sort of statin people used.

For example, there was no reduction in risk for black men taking any type of statin, and Atorvastatin (Lipitor) was associated with reduced risk among white, black, and Hispanic women, and Hispanic men only.

Pravastatin (Pravachol) and rosuvastatin (Crestor) were both associated with a reduced risk for only white women.

Writing in the medical journal JAMA Neurology, the researchers from the University of Southern California suggested that "the right type of statin, for the right person, at the right time, may provide an inexpensive means to decrease the burden of AD (Alzheimer's disease)".

"We may not need to wait for a cure to make a difference for patients currently at risk of the disease.  Existing drugs, alone or in combination, may affect Alzheimer's risk," said lead author of the study Julie Zissimopoulos.

Associate Professor Michael Woodward, a dementia expert based at the Austin Hospital and Melbourne University, said the study was very exciting because statins are among the most commonly prescribed drugs in Australia.

"If we could reduce Alzheimer's by even 10 per cent, we would reduce the number of cases by tens of thousands in Australia," he said.

However, he said the research only pointed to an association between the two, not a cause and effect relationship, so people should not rush out to get statins if they don't need them.

Associate Professor Woodward, the chief medical advisor to Alzheimer's Australia's Victorian branch, said more research was required because other factors could have been responsible for some people having a lower risk of dementia in the study.

"For instance, it might be that it's more socially and educationally privileged people who see their doctors more regularly and get put on statins compared to people from lower socio-economic groups," he said.

"We know that lower socio-economic class and low education are risk factors for Alzheimer's disease, so the statins might be marking people who have a lower risk of Alzheimer's disease for another reason."

The American research comes after some statin users reported memory loss as a side effect of the drugs. However, Australia's Therapeutic Goods Administration says the number of cases is low and for most people the symptoms reversed when the statin was stopped.

In 2012, the TGA said: "Recent reviews of available data have looked closely at this possible side effect and conclude that it is rare".

There are more than 350,000 Australians living with dementia and 1.2 million people care for somebody with the condition. One in 10 people aged over 65 have dementia and 3 in 10 people aged over 85 have it.

One of the researchers involved in the American study was funded by drug company Amgen.