The herbal preparation is extracted from the ginkgo biloba tree
A herbal extract used by an estimated 10% of people with dementia is not an effective treatment for it, an Imperial College London study suggests.
Ginkgo biloba is commonly marketed as an aid to memory and some studies have reported benefits.
But a six-month trial of 176 people with mild to moderate dementia found no difference between those taking ginkgo biloba and those taking a placebo.
The results are in the International Journal of Geriatric Psychiatry.
The researchers tested 120 mg daily of ginkgo biloba in patients recruited from London general practices.
They measured participants cognitive skills and quality of life at two, four and six months.
Tests included exercises such as recalling words from memory or answering questions about time or places.
There was no evidence that the standard dose of the herbal extract had any benefit on memory.
And quality of life as reported by the patient or by their carer did not improve over the course of the research.
A Cochrane review published in 2002 which had taken all relevant evidence into account found ginkgo may be a small beneficial effect on memory in dementia patients.
But the researchers say that is being updated in view of this latest research and has found that it is unlikely to provide benefits.
Study leader, Dr Rob McCarney who carried out the research at Imperial College London, said the lack of drug treatments in early dementia meant patients would try anything to slow down the progression of the disease.
"This isn't a hugely expensive treatment but if you're living on a state pension it can make a considerable dent in your budget.
"The findings add to the growing evidence that ginkgo provides no benefit."
He added that negative findings, such as these were in general less likely to be published.
"But we think this is equally important because people can clutch at straws when a diagnosis of dementia is made."
Professor Clive Ballard, director of research at the Alzheimer's Society, said the finding was very disappointing.
"However, this is an extremely important finding.
"Thousands of people with dementia, who are already struggling to make ends meet, may buy ginkgo biloba expecting an improvement in their memory."