A drug used to treat the symptoms of Alzheimer's disease may also help delay the onset of the condition, US scientists say.
Some 400,000 Britons have Alzheimer's disease
Donepezil is used by people with Alzheimer's to improve memory and help with physical functioning.
The team from the Mayo Clinic in Minnesota found the drug can help slow the progression from mild cognitive problems to full-blown Alzheimer's.
But UK experts said the drug may just be masking the disease's progress.
There is currently no way to prevent or slow the development of the degenerative brain disorder, which affects 400,000 Britons.
Lead researcher Dr Ronald Peterson said: "This may be the sign of new horizons to come in attempting to alter the Alzheimer's disease process as early as possible, buying time for those who may later progress."
But he said the concept, rather than the effect of the drug, was the most important aspect of the study, published in the New England Journal of Medicine.
"This study may be the front-runner in shifting our sights toward earlier treatment of the Alzheimer's process, laying the groundwork for testing other drugs."
The researchers gave 769 people with memory problems either vitamin E, donepezil or a placebo, an inactive pill, over three years.
Those given donepezil progressed at a slower rate - in some cases they were able to ward off the disease for up to three years - although they did eventually develop Alzheimer's.
The researchers suggested donepezil could be made available to people with mild cognitive problems.
But UK campaigners cast doubt on the findings.
Alzheimer's Research Trust chief executive Rebecca Wood said while the benefits of the findings were only short-term it was still "encouraging".
But she said the results did not "necessarily prove any slowing" of the disease as the symptoms may just have been hidden by the drug.
"The key to tackling dementia could be finding ways to diagnose Alzheimer's accurately in the early stages and then give future treatments that halt the disease before the brain is damaged."
However, she added the findings supported the trust's opposition to proposals by the National Institute for Clinical Excellence, the NHS advisory body, to withdraw donepezil along with other Alzheimer's drugs on cost-effectiveness grounds.
Professor Clive Ballard, director of research at the Alzheimer's Society, said developing treatments to prevent the onset of the disease was a "major research priority".
But he pointed out that neither vitamin E or donepezil prevented Alzheimer's.