A large study has shown the dietary supplement ginkgo didn't help prevent dementia and Alzheimer's disease.

Extracts from ginkgo tree leaves have antioxidant and anti-inflammatory effects, but earlier research on ginkgo and memory showed mixed results. Proponents claim gingko protects the brain by preventing the buildup of an Alzheimer's-related protein or by preventing cell-damaging oxidative stress.

To test the theories, researchers recruited more than 3,000 people, ages 75 and older. Half were randomly assigned to take 120 milligrams of ginkgo biloba twice a day. The others took identical dummy pills.

After six years, dementia had been diagnosed at a similar rate in both groups. Researchers noted a similar rate of Alzheimer's disease. The leader of the federally funded study said he doesn't think ginkgo "has a future as a powerful anti-dementia drug."

The study appears in tomorrow's Journal of the American Medical Association.