Q. Some years ago, you published an article saying ginkgo biloba might slow down dementia. I was 59, and I started taking it. I think it has helped me and know others who think likewise.

But I read that a recent study says it doesn't help. Your thoughts?

A. Ginkgo biloba -- or just ginkgo, for short -- is an herbal supplement made from the fan-shaped leaves from the tree of the same name. Ginkgo trees -- maidenhair is another name -- are native to China, but they can now be found all over the world, including the United States.

Ginkgo is prescribed in many parts of the world as a memory remedy and is readily available in the United States as an over-the-counter herbal supplement. It does have some antioxidant properties that theoretically could prevent the sort of brain damage that leads to memory problems, and there are hints from test-tube experiments that ginkgo might combat the formation of amyloid plaques, which are thought to be an underlying cause of Alzheimer's disease. Early trials showed that people who already had dementia had modest cognitive gains after taking ginkgo, although I think those results were unconvincing.

The randomized trial you're referring to was designed to test whether ginkgo might prevent or delay dementia, not reverse it once it has occurred. Sponsored by the National Institutes of Health, the study included about 3,000 older Americans (average age 79) who scored well on standardized tests for cognition, although a sizable minority (about 15 percent) were classified as having mild cognitive impairment, a less severe form of dementia that sometimes develops into a full-fledged case. Half were randomized to take a 120-mg dose of ginkgo twice a day -- a dose that previous studies have suggested might be effective -- while the other half took placebo pills.

After an average follow-up period of six years, the number of people who developed dementia was about the same in the ginkgo group (277 of 1,545, or 18 percent) as it was in the placebo group (246 of 1,524, or 16 percent). The results were published in The Journal of the American Medical Association in November 2008. Because the study was very large, and involved repeated measurements of cognitive function, I think the disappointing results are true: ginkgo did not prevent dementia among these study participants.

What does that mean for you?

In thinking about the results of any randomized trial, you need to ask whether the study subjects were like you and, therefore, whether the results apply to your situation. The people in this study were 20 years older, on average, than you were when you started taking ginkgo.

Theoretically, ginkgo might help people fend off dementia if they start taking it in their late 50s, as you did. But we don't have any proof of that. We'd need results from a randomized trial that was as well conducted as this one in people of that age.

There's something else to keep in mind in evaluating such studies: The overall results for any trial are just a statistical average.

The results for each individual in a study can be all over the map. So it's theoretically possible that some older people might benefit from ginkgo, even though the average results didn't show any benefit.

More research into genetic variants that increase beneficial effects of ginkgo might help us pinpoint those individuals. We're already learning that some people have gene variants that alter how they react to particular medicines.

The same might be true for ginkgo.

So what should you do in the meantime?

Ginkgo supplements don't seem to have many adverse effects, so there's probably little, if any, harm in taking them, although they can increase your risk of bleeding if you take clopidogrel (Plavix); heparin; a nonsteroidal anti-inflammatory drug like ibuprofen; or warfarin (Coumadin).

Also, no agency monitors the manufacture of supplements like ginkgo, so the purity and the actual dose in each pill are somewhat in question.

And remember, just because you haven't developed dementia while taking ginkgo doesn't mean the supplement deserves the credit.

There's a story of a man who carried horse chestnuts in his pocket for a decade to ward off rheumatism, and he remained free from the disease.

"And had you been greatly troubled with rheumatism before that?" a skeptical friend asked. "No, and that's the most remarkable thing of all," he replied, "it's retroactive."

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