There is debate among scientists and doctors about the "real world" impact of lecanemab.

The slower decline with the drug was noticed using ratings of a person's symptoms. It's an 18-point scale, ranging from normal through to severe dementia. Those getting the drug were 0.45 points better off. Prof Spires-Jones said that was a "small effect" on the disease, but "even though it is not dramatic, I would take it".

Dr Susan Kohlhaas, from Alzheimer's Research UK, said it was a "modest effect... but it gives us a little bit of a foothold" and the next generation of drugs would be better.

A crucial question is what happens after the 18 months of the trial, and the answers are still speculation.

Dr Elizabeth Coulthard, who treats patients at North Bristol NHS Trust, says that people have, on average, six years of living independently once mild cognitive impairment starts.

"Slow that decline by a quarter and it could equate to an extra 19 months of independent life, "but we don't know that yet", she says.

The drugs have to be given early in the disease before too much damage to the brain is done, whereas most people referred to memory services are in the later stages of the disease.

That requires people coming forward at the earliest signs of memory problems and doctors being able to send them for amyloid tests - either brain scans or spinal fluid analysis - to determine if they have Alzheimer's or another form of dementia.

The Alzheimer's Society says more than 850,000 people in the UK have some form of dementia. More than half have Alzheimer's, but everyone would need testing.

Kate Lee, chief executive of Alzheimer's Society charity, called for a 10-year government strategy on dementia to deal with what she called the "biggest health crisis we face in the UK".

Speaking to Radio 4's Today programme, she also said Lecanemab would not have a "huge impact" on those who already live with dementia. But she added it should "make a big difference" for future generations.