The first drug appearing to slow the destruction of brain in Alzheimer disease has been hailed as momentous and historic, BBC reports.
The medicine, lecanemab, has only a small effect and its impact on people's daily lives is debated, yet it works in the early stages of the disease, so most would miss out without a revolution in spotting it.
The limited data has generated excitement among dementia scientists and charities as the research breakthrough ends decades of failure and shows a new era of drugs to treat Alzheimer's - the most common form of dementia - is possible.
Lecanemab attacks the sticky gunge - called beta amyloid - that builds up in the brains of people with Alzheimer's.
For a medical field littered with duds, despair and disappointment, some see these trial results as a triumphant turning point.
Alzheimer's Research UK said the findings were "momentous".
One of the world's leading researchers behind the whole idea of targeting amyloid 30 years ago, Prof John Hardy, said it was "historic" and was optimistic "we're seeing the beginning of Alzheimer's therapies". Prof Tara Spires-Jones, from the University of Edinburgh, said the results were "a big deal because we've had a 100% failure rate for a long time".
Currently, people with Alzheimer's are given other drugs to help manage their symptoms, but none change the course of the disease.
Lecanemab is an antibody - like those the body makes to attack viruses or bacteria - that has been engineered to tell the immune system to clear amyloid from the brain.
Amyloid is a protein that clumps together in the spaces between neurons in the brain and forms distinctive plaques that are one of the hallmarks of Alzheimer's.
The large-scale trial involved 1,795 volunteers with early stage Alzheimer's. Infusions of lecanemab were given every fortnight.
The results, presented at the Clinical Trials on Alzheimer's Disease conference in San Francisco and published in the New England Journal of Medicine, are not a miracle cure. The disease continued to rob people of their brain power, but that decline was slowed by around a quarter over the course of the 18 months of treatment.
The data is already being assessed by regulators in the US who will soon decide whether lecanemab can be approved for wider use. The developers - the pharmaceutical companies Eisai and Biogen - plan to begin the approval process in other countries next year.
David Essam, who is 78 and from Kent in the UK, took part in the international trial.
His Alzheimer's meant he had to give up work as a joiner - he could no longer remember how to build a cabinet or use his tools. He now uses a digital watch as he can't tell time using a clock face.
"He's not the man he was, he needs help with most things, his memory in general is almost non-existent," said his wife Cheryl. But she said the trial had given the family hope.
David said: "If somebody can slow it [Alzheimer's] down and eventually stop it all together that would be brilliant, it's just a horrible nasty thing."
There are more than 55 million people in the world like David and the numbers with Alzheimer's disease are projected to exceed 139 million by 2050.