Brain deterioration caused by Alzheimer's has been delayed with a drug for the first time, researchers have announced.
Scientists trialling a new treatment called LMTX managed to slow the progression of the disease over 15 months.
The study found that when the drug was the only treatment patients took, it had a beneficial effect on key measures of Alzheimer's - such as memory - for those with mild or moderate forms of the disease.
The small trial of 136 patients saw participants take the drug, leuco-methylthioninium-bis (hydromethanesulfonate), as a pill twice a day.
However, in a bigger trial, patients taking other drugs alongside LMTX did not see the same benefit.
Serge Gauthier from McGill University in Canada presented the results at the Alzheimer's Association International Conference in Toronto.
He said: "In a study of this size across a combined mild to moderate patient population, it is both encouraging to see improvements of this magnitude in the standard cognitive and functional tests and reassuring to see the supporting brain scan evidence of a slowing in disease progression during 15 months of treatment.
"As a practising clinician I see Alzheimer's patients, their families and care-givers every day, and continually share their desperate need for a truly therapeutic product as today we only have symptomatic treatments available to us.
"In a field that has been plagued by consistent failures of novel drug candidates in late-stage clinical trials and where there has been no practical therapeutic advance for over a decade, I am excited about the promise of LMTX as a potential new treatment option for these patients."
Speaking to The Times he added: "This is the first time it has happened in our field that a drug reduced the rate of brain atrophy."
Claude Wischik, from the University of Aberdeen, invented the drug and told the newspaper that he hoped to apply for a licence after publishing the results of a second trial.
LMTX (also called LMTM) is based on the structure of a chemical called methylene blue which is used as a dye in research and to aid surgery.
It works by tackling problems linked to proteins called tau that occur in Alzheimer's sufferers.
The chief scientific officer at Alzheimer's Research UK said the results were the "silver lining" of a study which did not meet its main goals.
David Reynolds said: "The idea that a drug based on a common blue dye could target one of the key processes driving Alzheimer's was suggested well over 10 years ago, so it's positive to see LMTM being tested in people.
"The build-up of abnormal tau in nerve cells is a hallmark of Alzheimer's and several other dementias, so there has been much hope about what potential benefit a drug against this process could bring for people with dementia.
He added: "While today's announcement marks an important step in the evolution of Alzheimer's clinical trials, we must be cautious in our interpretation until questions raised by the trial have been explored further.
"The positive findings will need to be confirmed in the second ongoing phase III trial before it's possible to assess whether LMTM as a monotherapy could provide more benefit to patients than current Alzheimer's drugs."