This is a bittersweet moment for people living with Alzheimer's disease and their families.
Hopes had been high that for the first time they would have access to a treatment that could slow the decline in memory and thinking.
When a pivotal clinical trial was released last year doctors hailed lecanemab as a game-changer, one that could delay the need for care and give families more time together.
But while the medicine's regulator has agreed that the drug can be prescribed, the NHS spending watchdog has ruled in draft guidance that it is too expensive to justify the benefits.
Unusually that isn't just because of the cost of the drug.
It's also the resource implications for the NHS of organising fortnightly infusions for so many people. And then there is the careful diagnosis and intensive monitoring that is needed to keep patients safe.
Lecanemab is a potent drug. It clears the clumps of abnormal protein called amyloid that build up in the brains of people with Alzheimer's.
But in doing so it can sometimes cause dangerous swelling and bleeding in the brain.
So doctors need to be sure patients really have Alzheimer's before they start treatment.
That can only be done at the moment with expensive PET brain scans or samples of spinal fluid - and the NHS doesn't have enough of the right machines or staff.
And patients also need regular checks with an MRI scanner to check there are no side effects in the brain.
All in all, the National Institute for Health and Care Excellence (NICE) said it wasn't worth all the resources just to delay the slide from mild to moderate disease by about four to six months.
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The decision applies to England - Wales and Northern Ireland often follow that lead. Scotland's spending watchdog has yet to make a decision.
So where does this leave patients?
Those who have the funds will be able to pay for private care. In the US that's upwards of £20,000 a year. It will feel unfair to many that availability is only there for the privileged.
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But some doctors argue that although the drug was a milestone - the first to have an effect - the effect was relatively modest.
A healthy lifestyle can also slow the progress of the disease - a good diet, lots of exercise, not smoking and maintaining social connections are really important.
Decades of research are beginning to pay dividends. Lecanemab is the first of many drugs coming down the tracks. Each will have to be judged on its merits.
But sometime soon it will be possible to give people the hope of treatment.