The supplements’ appeal is understandable. A growing older population with longer life spans means more people with dementia, though in population-based studies in this and other Western countries, its prevalence has fallen.

More of us have seen the devastation up close and would do almost anything to evade it. But so far, the news about drugs and supplements has been discouraging.

Although scientists have learned much more about dementia, the research literature and large pharmaceutical trials have mostly served to tell worried Americans about the many substances that don’t appear to prevent, treat or slow dementia.

Vitamins, various antioxidants, concoctions derived from animals and plants — “we see plenty of ads on TV, but we have no evidence that any of these things are preventive,” said Dr. Steven DeKosky, a neurologist and deputy director of the McKnight Brain Institute at the University of Florida.

Dr. DeKosky led a federally supported study of Ginkgo biloba extract, for instance, following more than 3,000 people for seven years to see if it reduced dementia. It didn’t.

“No effects at all,” he said. “But look on the shelves. Many companies still sell ginkgo — if there’s really any in there, because supplements don’t always have the contents they say they have.”

Moreover, “some of these supplements are biologically active and can cause toxicity when you take other drugs,” said Dr. DeKosky. Supplements can be costly, too.