A Chinese herb called Ginkgo biloba billed by some as a potential over-the-counter wonder drug that boosts mental dexterity and sharpens the memory has been found to do nothing to prevent the onset of Alzheimer’s disease. A large scientific study of the supplement by researchers in France found that people who took twice-daily doses of Ginkgo biloba were no less likely to develop the brain-wasting disease than those given a placebo, or dummy pill. The study, published in The Lancet Neurology journal on Thursday, suggests anyone taking the herb extract in the hope of escaping Alzheimer’s is wasting their money, experts said.