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

Lecanemab has been approved for use in the UK by the UK Medicines and Healthcare products Regulatory Agency (MHRA), meaning it can be prescribed privately.

In experimentsThe drug was shown to slow cognitive decline in patients with early-stage Alzheimer’s disease by about a quarter over an 18-month period.

But in the draft guidelinesNICE said the treatment entails significant costs, including intensive monitoring for side effects and two-week hospital stays for patients.

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

“The approval of lecanemab is a milestone, but NICE’s decision not to approve it for the NHS is deeply disappointing,” said NICE’s head of policy, David Thomas.

Dr Samantha Roberts, chief executive of NICE, said the organisation had carefully reviewed the available evidence, including the benefits for carers, but could only recommend treatments that offered “good value for the taxpayer”.

“Lecanemab slows the progression of the disease from mild to moderate Alzheimer’s by an average of four to six months. However, this benefit is not sufficient to justify the additional cost to the NHS,” said Helen Knight, head of drug evaluation at NICE.

A final decision by NICE will be made after a public consultation towards the end of the year.

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

Wales and Northern Ireland often follow England’s medical guidelines.

The panel that assesses newly approved medicines in Scotland has not yet made a decision on the drug’s value.

According to NHS England, a dedicated team is currently reviewing 27 other Alzheimer’s drugs that are currently in advanced trials and could be approved in the coming years.

Lecanemab works by eliminating an unwanted protein called amyloid that builds up in the brains of Alzheimer’s patients. It is given as an intravenous infusion every two weeks.

Alzheimer’s researchers described the study results as historic because none of the previous drugs had convincingly demonstrated that the underlying mechanism of the disease could be slowed down.

However, there have been concerns about the common occurrence of so-called amyloid-related imaging abnormalities (ARIAs) that can be seen in MRI scans, such as small brain bleeds and temporary swelling.

Although most of these cases in the studies were mild or asymptomatic, some cases required hospital treatment.

No price for the drug has yet been publicly announced in the UK, but in the US it costs around £20,000 per patient per year.

Fiona Carragher of the Alzheimer’s Society told BBC News the decision would “lead to uncertainty and confusion for the nearly one million people living with dementia”.

Although the MHRA has approved the drug in the UK (England, Wales and Scotland), it has placed some restrictions on the patients who can receive it due to the risk of side effects.

People who carry two copies of the apolipoprotein E4 (ApoE4) gene (about 15% of Alzheimer’s patients) are not eligible to participate. This also applies to people who take blood thinners.

Lecanemab, whose brand name is Leqembi, is already approved in the USA, Japan and China.

But last month, the European Medicines Agency (EMA) a license was refused, He said the benefits were small and did not outweigh the risk of serious side effects, particularly bleeding and swelling in the brain.

To assess whether a patient is suitable for lecanemab, the level of amyloid in their brain must be measured. This is done either via a PET brain scan or a lumbar puncture.

None of these procedures are a standard diagnostic tool for patients suspected of having dementia and they are only used in research contexts. It is estimated that only 2% of Alzheimer’s patients have access to one of these ‘gold standard’ diagnostic methods.

Mavis Guinn, 90, is one of only a few dozen people in the UK receiving lecanemab as part of a clinical trial.

Her husband Rodney says it has allowed Mavis to maintain her personality.

“We’ve had some wonderful years and great times since you started taking that medicine,” he tells her.

“And I’m grateful for it too, my goodness, because it makes a difference in your life, doesn’t it,” says Mavis.

She receives the medication via infusion every two weeks, but her short-term memory remains severely impaired.

The benefits of lecanemab are modest – some even say they are barely noticeable. Then there are the potential side effects.

Yet many experts see this as a defining moment that shows that Alzheimer’s is not unstoppable. Alzheimer’s is the most common form of dementia and the leading cause of death in the UK.

The BBC magazine Panorama followed patients taking lecanemab and another new drug, donanemab.

In the program, In a programme broadcast earlier this year, Prof Cath Mummery, a consultant neurologist and director of clinical trials at the Dementia Research Centre at University College London, said that while the benefits of the drugs were small, they represented a game-changer.

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