Alzheimer's drugs currently being denied to some NHS patients may have a dramatic impact on the pathology of the brain, research in the UK indicates.
The drugs are reserved for those with moderate Alzheimer's
Neurology says that post-mortem tests on 24 patients found a 70% fall of a protein linked to dementia in those who had taken cholinesterase inhibitors.
Campaigners are fighting to get the drugs available on the NHS after they were rejected for use in mild cases.
Experts said the study was interesting but small and inconclusive as a result.
Post-mortem examinations were done on 12 patients who took part in UK trials of the drugs - donepezil, rivastigmine, tacrine and galantamine.
Professor Clive Ballard, director of research at the Alzheimer's Society, and colleagues measured the concentrations of two proteins associated with the build up of plaques found in the brains with people with dementia.
The results were compared with 12 patients studied before cholinesterase drugs were available.
Deposits of one of the proteins - beta amyloid - in the plaques were 70% lower in the brains of people who received the drugs in the trial.
There was no difference in amounts of the other protein known as tau.
Professor Ballard said: "We knew there may be some reduction in the levels of beta-amyloid among people prescribed cholinesterase drugs, but the sheer magnitude of the reduction was a real surprise.
"The study looked at dementia with Lewy bodies but beta-amyloid is also a hallmark in Alzheimer's disease.
"The results suggest that if we want to slow down the progression of these diseases the earlier we start prescribing these treatments the better."
Guidance from the National Institute for Health and Clinical Excellence (NICE) in 2001 recommended donepezil, rivastigmine and galantamine should be used as standard.
But in November the watchdog announced people with newly diagnosed, mild Alzheimer's were exempt.
The Alzheimer's Society is set to go to the High Court next month as part of a judicial review of the decision.
Neil Hunt, chief executive of the Alzheimer's Society, added: "People with Alzheimer's disease and their carers have known about the life-changing benefits of these drugs for some time now and this study provides the first hard evidence of the physical benefits of the same treatments."
But Dr Declan McCloughlin, senior lecturer in the Medical Research Council Centre for Neurodegeneration, said there could be other explanations for the findings.
"This is a very interesting but preliminary finding which could have happened by chance because the sample groups are small.
"We know from clinical trials the drugs have modest benefits and so if they have an effect on the underlying molecular pathology it's not a very big effect.
"But is shows there may be some improvement in one of the key pathologies of the disease."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "Although the scientists looked at Lewy Body disease rather than Alzheimer's, the two conditions have much in common and this study shows how a class of drugs, restricted by NICE for so-called performance reasons, do help to prevent the physical progression of dementia."