A Chinese herb claimed to improve memory and mental sharpness is not able to prevent Alzheimer's disease, scientists have found.
Researchers in France tested the effect of Ginkgo biloba on more than 2,800 elderly volunteers who took the supplement over five years.
Half the participants, who were all aged 70 or older, were given twice-daily doses of Ginkgo and half received an inactive placebo.
During the trial, rates of dementia and Alzheimer's symptoms hardly differed between the two groups.
A total of 61 people who were given Ginkgo were diagnosed with Alzheimer's - the most common dementia disease - compared to 73 people in the placebo group.
Alzheimer's Society spokeswoman Jess Smith said: "It is understandable that people would want to hear that an over-the-counter herbal remedy could be the answer to preventing Alzheimer's.
"For a while it was hoped that Ginkgo biloba could be the wonder drug.
"However, in recent years evidence - including a previous study by Alzheimer's Society - has repeatedly shown that it does not have any benefits in preventing the disease or slowing down symptoms.
"One in three people over 65 will develop dementia. The best way for people to reduce their risk is to eat a healthy diet, exercise regularly, not smoke and keep their cholesterol levels in check."
Researchers also found no significant difference between the number of people who died or developed a stroke.
Results from the study by the University of Toulouse were published today in The Lancet Neurology. It is only the third Alzheimer's prevention trial to be completed and the first to be done outside the USA.
Many people use Ginkgo leaf extracts to improve memory or treat or prevent Alzheimer's and other types of dementia.
Other ways the herb is used include treating sexual dysfunction, multiple sclerosis and leg pain caused by narrowing arteries.
In Europe and the US, Ginkgo supplements are among the best-selling herbal medicines.
One in three people over 65 will die with dementia, the Alzheimer's Society said.
About 800,000 people in the UK have a form of dementia, with more than half diagnosed with Alzheimer's disease.
It is estimated that one million people will be living with dementia in a decade's time, with 1.7 million people diagnosed by 2051.
Professor Bruno Vellas, lead author of the study, said: "While our trial appears to have shown that regular use of ginkgo biloba does not protect elderly patients from progression to Alzheimer's disease, more studies are needed on long-term exposure.
"The fact that prevalence of this debilitating disorder is expected to quadruple by 2050 suggests that research into preventative therapies for this disease needs to receive urgent attention."