The drugs cost about £2.50 a day
The Appeal Court has ruled an NHS advisory body should have been more transparent in the way it made decisions over Alzheimer's drugs.
Three judges said NICE should have released details of how it reached its decision to limit drugs like Aricept to people with late stage disease.
Its maker Eisai had challenged NICE's refusal to release the information.
However access to the drugs will remain limited as the court did not overturn NICE's ruling on their use.
The National Institute for Health and Clinical Excellence (NICE), has concluded drugs are not cost-effective in early disease.
But Eisai claimed it should have been able to see the detail of how that decision was reached, saying almost 100,000 patients a year with early-stage disease were being denied access to the medication.
In the Appeal Court ruling, Lord Justice Richards said withholding information put drugs companies at "a significant disadvantage" if they wanted to challenge a NICE ruling.
Eisai will now be able to assess NICE's cost-benefit analysis.
Its comments will have to be considered by the drugs body, which may then have to review its decision.
Other drug appraisals are also likely to be affected.
Nick Burgin, managing director of Eisai, said: "We believe that this decision represents a victory for common sense.
"We hope that this action will ultimately restore access to anti-dementia medicines for those patients at the mild stages of Alzheimer's disease."
The company brought its case with support from fellow drugs firms Pfizer and Shire, which manufacture other drugs affected by the NHS ruling, and the Alzheimer's Society.
John Young, Pfizer's managing director, said: "Contrary to NICE's position that they follow a fully fair and transparent process, the Court of Appeal found that this is not the case.
"The failure to disclose these fundamentally important calculations has impaired the ability of stakeholders to engage fully in the appraisal process."
The Association of the British Pharmaceutical Industry (ABPI) said the Appeal Court's decision would allow greater scrutiny of how NICE worked.
Neil Hunt, chief executive of the Alzheimer's Society said the decision was a damning indictment of the "fundamentally flawed process" used by NICE.
He urged it to review its ruling on the drugs and make them more widely available.
Andrew Dillon, chief executive of NICE, said: "We will be considering very carefully the findings and the implications for the time it takes us to provide advice to patients and the NHS on the use of new treatments.
"The ruling will increase the complexity of our drug appraisals in some cases and they may take longer as a result."
NICE guidance in 2001 recommended the drugs - which can make it easier to carry out everyday tasks - should be used as standard.
But advice published in November 2006, stated that the drugs should only be prescribed to people with moderate-stage disease.
NICE said the drugs, which cost about £2.50 a day, did not make enough of a difference to recommend them for all patients and were not good value for money.