A Chinese herb claimed to improve memory and mental sharpness is not able to prevent Alzheimer's disease, scientists have found.

Researchers in the US tested the effect of Ginkgo biloba on more than 3,000 elderly volunteers who took the supplement for several years.

Half the participants, who were all aged 75 or older, were given twice-daily doses of Ginkgo and half received an inactive placebo.

During the trial, 523 of the volunteers were diagnosed with dementia, 92% of whom were thought to be suffering from Alzheimer's - the most common dementia disease.

Rates of dementia and Alzheimer's symptoms hardly differed between the two groups.

Ginkgo also had no effect on the progression of dementia in participants with mild cognitive impairment.

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.

The Ginkgo biloba for the Evaluation of Memory (GEM) study was the largest clinical trial ever undertaken to assess the effects of Ginkgo on dementia.

Dr Jeff Williamson, from Wake Forest University Baptist Medical Center in Winston-Salem, New Carolina, said: "It is very unlikely that Ginkgo biloba is effective at any dose over a five-year period and in anyone over 75 years old.

"It is also ineffective in people with signs of early memory loss. What is not known yet is whether the effect of Ginkgo biloba might require taking the drug for many, many years, say 15 years, before there is even a sign of memory loss."

He urged the millions of people who buy Ginkgo to protect themselves against Alzheimer's to spend their money elsewhere.

Results from the trial were reported today in the Journal of the American Medical Association.

In an accompanying editorial, Dr Lon Schneider, from the University of Southern California, Los Angeles, wrote: "Despite two decades of research with standardised extracts of Ginkgo biloba, considerable uncertainty about its pharmacology and clinical effects remains.

"Preclinical scientific reports exude promise but generally have not identified the relevant active molecules of this biochemically complex extract, and the preclinical promise has not translated to clinical research benefits."