A turning point in the fight against Alzheimer's may have been reached with the first trial evidence that progression of the disease can be held back.

The experimental drug solanezumab, made by US pharmaceutical giant Eli Lilly, reduced mental decline by 34 per cent in a group of patients taking a standard battery of memory and thinking tests.

It is the first time an Alzheimer's drug has been shown to have a "disease modifying" effect rather than merely alleviating symptoms.

The findings offer a glimmer of hope to victims of the devastating disease suffered by an estimated 500,000 people in the UK, including 90,000 in Scotland.

But only patients with mild Alzheimer's who started treatment early experienced the beneficial effect over a period of three and a half years.

The news has been welcomed by Alzheimer Scotland, which said the initial tests sound promising. A spokesman said they indicated that the compound has some effect, but they would have to await A follow up trial – which will not be for 18 months or more – before it would be possible to measure the influence of this treatment.

He added: "More promisingly, it suggests that drug treatments can have an impact on brain function if a person’s amyloid pathologies are identified early enough, which bodes well for other research – such as EPAD at the University of Edinburgh."

Neuroscientist professor Richard Morris, from the University of Edinburgh, said: "I am cautiously optimistic. This is not a mouse study, it's a people study. And that matters."

The results were presented at the Alzheimer's Association International Conference in Washington, where another drug was shown to shrink protein deposits in the brain linked to Alzheimer's.

Again, the effect was seen in people at an early stage of the disease. An interim safety study found that the drug aducanumab reduced the size of beta amyloid plaques increasingly as the dose was raised.

Both drugs are laboratory-made antibodies that target specific proteins, in this case sticky clumps of beta amyloid.

Dr Doug Brown, head of research at Alzheimer's Society, said: "Today's findings strongly suggest that targeting people in the earliest stages of Alzheimer's disease with these antibody treatments is the best way to slow or stop Alzheimer's disease.

"These drugs are able to reduce the sticky plaques of amyloid that build up in the brain, and now we have seen the first hints that doing this early enough may slow disease progression.

"After a decade of no new therapies for dementia, today's news is an exciting step forward.

"We will have to wait for the ongoing trials to finish to know the full risks and benefits of these drugs. If they are positive, these drugs will be the first identified to directly interfere with the disease process and slow the progression of Alzheimer's."

A total of 1,024 patients with an average age of 73 took part in the solanezumab investigation. Researchers pooled data from two earlier trials, Expedition and Expedition 2, and a later extension study involving the same participants.

Initial results from the Expedition trials, published in 2012, suggested that the drug - like a number of other experimental medicines showing early promise - was a failure. Treated patients fared no better than those given a dummy placebo.

But the picture changed when researchers picked out a subset of patients with mild symptoms who had started treatment early. These patients experienced a benefit that was sustained over time, so that those on the drug for 3.5 years were better off than others who had only taken it for two years.

This was evidence that the drug was treating the disease itself, not just its symptoms. However, some experts have urged caution.