NHS patients are set to miss out on new Alzheimer's drugs, experts warn.
Earlier this year lecanemab, the first treatment found to slow disease progression, was licensed by regulators.
However, the NHS rationing body - the National Institute for Health and Care Excellence (Nice) - ruled in draft guidance that the benefits are not worth the costs.
A second treatment donanemab is expected to suffer the same fate shortly.
But with almost 30 similar drugs in the pipeline, leading Alzheimer's charities have urged the NHS to introduce a system that would track people's treatment over time in order to benefit future patients.
They say it is a matter of "when, not if" the first drugs are rolled out on the NHS, and a mandatory data collection system would help experts to learn more about the new medicines. This would then speed up approval of future drugs.
Scientists said the NHS needs to capture real world data to establish which patients benefit most from the treatments, how long it should be given for, and at what stages of the disease.
They have written to Jeremy Isaacs, the NHS national clinical director for dementia, calling for the kind of system which was introduced for innovative cancer treatments to also be used for dementia.
David Thomas, head of policy and public affairs at Alzheimer's Research UK, said: "Given the number of these drugs in the pipeline it is a matter of when, not if, regulators give the green light for the NHS.
"We need a national mandatory data collection agreement so that we can learn a lot more about how these drugs work outside of clinical trials - we need to track how these treatments affect people over time, when is the right time for them to be given and understand who benefits most."
So far, clinical trials have found that certain genetic variants increase the risk of side effects from the drugs.
People with two copies of the APOE4 Alzheimer's risk gene experienced more serious side effects - around 15 per cent of people with Alzheimer's disease carry two copies of this gene. Unlike other regulators worldwide, the Medicines and Healthcare products Regulatory Agency decided to exclude these people from the lecanemab licence.
The MHRA is expected to make the same exclusion if it gives donanemab the green light.
Collecting more data as part of a national platform could show how certain genetic make ups are impacted by the drug, guiding future clinical decisions about who should receive it.
Such findings could be crucial to understanding how the whole class of medication works, which could drastically speed up access to future drugs instead of them constantly failing at Nice hurdle, experts said. Lack of long-term data on safety and efficacy was one of the factors which led Nice to reject lecanemab.
Under a similar system used to log cancer drug data, some medicines are funded by the NHS while more data is collected. Between 2016 and 2023, this system helped support nearly 47,000 patients and informed treatment use across the NHS.
Alzheimer's Research UK also urged health officials to consider "managed access" to the breakthrough drugs: meaning that some patients could access the drugs on the NHS while more data was gathered.
An NHS England spokesman said: "While there is already a comprehensive and publicly available dataset that reports on those with a diagnosis of dementia, and this continues to be expanded with new information, the NHS is exploring further ways in which it can use routinely collected data to inform dementia care, and we welcome partnerships with patient groups and professional bodies as progress is made.