The first drug to slow the progression of early-stage Alzheimer’s will not be available on the NHS in England, because health assessment body NICE says the benefits are “too small to justify the cost”.

Lecanemab is licensed by the UK medicines regulator MHRA, meaning it can also be prescribed privately.

In trialsThe study found that the drug was successful in slowing cognitive decline in early-stage Alzheimer’s patients by about a quarter over a period of 18 months.

But in the draft guidance, NICE said the treatment would come at a significant cost, involving close monitoring for side effects and patients having to visit hospital every fortnight.

Alzheimer’s Research UK said it was “a bittersweet moment”.

“The approval of lecanemab is a significant moment, but the decision by NICE not to approve it for the NHS is extremely disappointing,” said policy lead David Thomas.

NICE chief executive Dr Samantha Roberts said the organisation had thoroughly assessed the available evidence, including the benefits for carers, but could only recommend treatments that “offer good value to the taxpayer”.

“Lecanemab provides an average four to six months’ slowing of the rate of progression of mild to moderate Alzheimer’s disease, but this is not a significant enough benefit to justify the additional cost to the NHS,” said Helen Knight, director of medicines evaluation at NICE.

Around 70,000 adults in England would have been eligible for treatment with lecanemab.

The body that assesses newly-licensed medicines in Scotland has not yet made a decision on the drug’s price.

Lecanemab works by clearing away a dangerous protein called amyloid that builds up in the brain of Alzheimer’s patients. It is given intravenously every two weeks.

Alzheimer’s researchers called the trial results landmark, as no drug had previously shown so clearly that the underlying mechanisms of the disease could be slowed.

But there were concerns over the frequent occurrence of amyloid-associated imaging abnormalities (ARIA), which show up on MRI scans, such as mild bleeding and temporary swelling in the brain.

Although symptoms were mild or asymptomatic in most of these trials, in some cases participants required hospital treatment.

The price of the drug in the UK is not publicly disclosed, but in the US it costs around £20,000 per patient per year.

Fiona Carragher, from the Alzheimer’s Society, told BBC News the decision would “cause uncertainty and confusion for around one million people living with dementia.”

Although the MHRA has approved this medicine in the UK, it has placed some restrictions on patients taking this medicine due to the risk of side effects.

People who have two copies of the apolipoprotein E4 gene (ApoE4) will not be eligible, nor will people who are taking blood-thinning medications.

About 15% of people have two copies of the APOE4 gene.

Lekanemab, whose brand name is Lekambi, has already been approved in the US, Japan and China.

But last month the European Medicines Agency (EMA) License denied They said the benefits were small and did not balance the risk of serious side effects, particularly bleeding and swelling in the brain.

To assess a patient’s eligibility for lecanemab, amyloid levels in their brain must be measured. This is done either through a PET brain scan or through a lumbar puncture.

None of these are standard diagnostic tools for patients with suspected dementia and are only used in research settings. It is estimated that only 2% of Alzheimer’s patients have access to one of these “gold standard” diagnostic methods.

The benefits of lecanemab are modest — in fact, some argue they are barely noticeable. Then there are the potential side effects.

Yet many in the field see it as a watershed moment, showing that Alzheimer’s is not impossible to stop. Alzheimer’s is the most common form of dementia and the leading cause of death in the UK.

The BBC’s Panorama looked at patients responding to lecanemab and another new drug, donanemab.

In ProgramIn an interview broadcast earlier this year, Professor Kath Mummery, consultant neurologist and head of clinical trials at University College London’s Dementia Research Centre, said although the drugs’ benefits were small, they represented a turning point.

“For the first time, we have drugs that show you can reverse the course of Alzheimer’s disease, and that’s an extraordinary thing.”