A Chinese herb called Ginkgo biloba billed by some as a potential over-the-counter wonder drug that boosts mental dexterity and sharpens the memory has been found to do nothing to prevent the onset of Alzheimer's disease.

A large scientific study of the supplement by researchers in France found that people who took twice-daily doses of Ginkgo biloba were no less likely to develop the brain-wasting disease than those given a placebo, or dummy pill.

The study, published in The Lancet Neurology journal on Thursday, suggests anyone taking the herb extract in the hope of escaping Alzheimer's is wasting their money, experts said.

"For a while it was hoped that ginkgo biloba could be the wonder drug," said Jess Smith, a spokeswoman for the Alzheimer's Society.

"However, in recent years evidence ... has repeatedly shown that it does not have any benefits in preventing the disease or slowing down symptoms."

The trial was conducted over five years and involved 2,854 people in France who were aged 70 years or older and who had already visited their doctors with concerns about their memory.

Ginkgo biloba extract - which is derived from the leaves of the Ginkgo biloba tree - was given to 1,406 patients, while the other 1,414 were given a placebo designed to have a similar taste and appearance to the Ginkgo pills.

Researchers then used standard tests to assess the patients' memory, cognitive function and dementia status.

After five years, 61 people in the ginkgo biloba group, or 4 percent, had been diagnosed with probable Alzheimer's disease, compared with 73, or 5 percent, in the placebo group.

The researchers, led by Bruno Vellas of the Hopital Casselardit in Toulouse, France, said the difference in results for the two groups was not statistically significant. The study also appeared to confirm the findings of a 2009 trial in the United States which had similar results.

NICE THEORY "DESTROYED BY UGLY FACT"

Vellas said that while the results suggested regular use of Ginkgo biloba did not protect the elderly from progression to Alzheimer's, more studies were needed to examine longer-term use of the herb and other potential prevention measures.

An estimated 18 million people worldwide suffer from Alzheimer's disease and as populations age, rates of the disease are expected to soar.

The fatal brain disease, which has no cure and few effective treatments, affects memory, thinking and behavior and is placing an increasingly heavy burden on societies and economies across the world.

"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," Vellas said in a statement.

Edzard Ernst, a professor and former director of complementary medicine at Britain's University of Exeter, said the study's findings were important, even if they were disappointing for those who want to believe in herbal remedies.

"This is by far the largest trial of Ginkgo so far," he said in an emailed comment. "The results are disappointing and fail to show that this herbal remedy reduces the risk of Alzheimer's. Another beautiful herbal theory destroyed by an ugly fact."