Ginkgo appears relatively safe, DeKosky said. There was no difference in the rate of adverse events such as heart attacks and gastrointestinal bleeding between the groups. There were 16 strokes from bleeding in the brain in the ginkgo group versus eight in the placebo group, a difference that wasn't statistically significant, he said, because the number of strokes was too small.
Would ginkgo work better to prevent dementia if people started taking it earlier, say, in middle age? The study didn't look at that, DeKosky acknowledged, adding that following people for 25 years from middle age to old age would be expensive research. The study also didn't test whether ginkgo improves thinking and memory in the short term.
"It would have been terrific if this worked. It's inexpensive, available and relatively safe," said Paul Solomon, professor of neuroscience at Williams College in Williamstown, Mass., who wasn't involved in the new study but has studied ginkgo.
"Now with this kind of evidence, you can confidently tell people it didn't show benefit in more than 3,000 people in six years of research," Solomon said.
There may still be a role for ginkgo in treating, rather than preventing, Alzheimer's disease, said Michael McGuffin, president of the American Herbal Products Association, a trade association.
Some previous ginkgo trials have shown no benefit in Alzheimer's symptom treatment, while others have found it comparable to prescription drugs such as Aricept, also known as donepezil.
A four-month supply of ginkgo can cost less than $10. But not all brands contain what their labels claim. ConsumerLab.com, which tests the ingredients in supplements, reported Tuesday that of seven ginkgo products it reviewed, five failed to pass its tests. An eighth ginkgo product also passed the group's test in a voluntary certification program.
The dementia study was funded by a grant from the National Institutes of Health's National Center for Complementary and Alternative Medicine, which is undertaking large, scientific tests of a number of commonly used dietary supplements. The agency said it's the largest-ever randomized trial of a botanical medicine. There have been larger studies of vitamins and minerals.
DeKosky reported receiving grants from and acting as a consultant for several drug companies, including some that make dementia drugs. Schwabe Pharmaceuticals of Karlsruhe, Germany, provided the ginkgo tablets and identical placebos.
People on the blood thinner warfarin shouldn't take ginkgo because of the risk of increased bleeding.