Popular Chinese herb ginkgo biloba which was viewed as a wonder drug to improve memory was proved ineffective in stopping Alzheimer's.
One of the longest and most rigorous studies ever conducted on ginkgo biloba found no proof that it helped to prevent the disease among older people starting to have memory problems, the Daily Mail reported.
Doctors claimed that this is the final nail in the coffin for the Chinese remedy, following other trials that have found no benefit.
Around 820,000 Britons are affected by dementia with symptoms such as memory loss, confusion and mood changes, with Alzheimer's the most common cause.
Many people with Alzheimer's as well as those concerned about becoming affected take gingko, derived from the leaves of the Ginkgo biloba tree.
The latest study by French researchers looked at whether the supplements could prevent the disease taking hold.
The trial took place over five years, involving 2854 people who were 70 years old or over, and who had asked their family doctor about memory problems.
Altogether 1406 patients were given a twice daily dose of 120mg ginkgo biloba extract, and 1414 given a placebo, designed to have a similar taste and appearance to the ginkgo pills.
Researchers used standard tests at regular intervals to assess the patients' memory, brain function and dementia status.
After five years, 61 (four percent) of those taking ginkgo had been diagnosed with probable Alzheimer's disease, compared with 73 (five percent) participants in the placebo group.
The difference was not statistically significant, and the researchers also found no significant difference between the groups in the number who had died or had a stroke.
Lead author Professor Bruno Vellas of the Hopital Casselardit, in Toulouse, France, said it was the largest ever Alzheimer's prevention study carried out in Europe.
"Our trial was unique in that the study population was made up of more than 2,500 individuals aged 70 years and older, who were free of dementia, and who had spontaneously reported subjective memory complaints," he 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," he added.
Ginkgo biloba is extracted from the leaves of the Ginkgo - the world's oldest living tree - which was left only in China after the Ice Age.
It contains three groups of complex compounds called flavonoids, diterpines and sesquiterpines which are thought to increase blood flow and improve circulation.
The study was published in the medical journal The Lancet Neurology.