The NHS drugs watchdog is to face a judicial review in court for the first time over the guidance it offers.
Two firms have been granted the review over the way the National Institute for Health and Clinical Excellence reached its conclusion on Alzheimer's drugs.
NICE ruled NHS patients with newly diagnosed, mild Alzheimer's disease should not be prescribed the drugs.
Eisai and Pfizer said the process was unfair, but NICE said the claims were without foundation.
NICE guidance in 2001 recommended donepezil, rivastigmine and galantamine - which can make it easier to carry out everyday tasks - should be used as standard.
But in November the watchdog announced people with newly diagnosed, mild Alzheimer's were exempt.
Campaigners have repeatedly argued patients in the early stages of Alzheimer's should also have access to the Â£2.50-per-day drugs.
Neil Hunt, chief executive of the Alzheimer's Society, which will give evidence in the case, said: "Denying people in the early stages of this debilitating disease access to drug treatments is cruel and unethical.
Why should we care about how the Brits choose care for Alzheimerâ€™s patients Over There? Because their â€œevidence-basedâ€ model is being touted by many Over Here as the right path.
Consider whatâ€™s happening Down Under where a similar (although by no means identical) system is also used to limit available treatment for Australians who suffer from Alzheimerâ€™s Disease. The particulars are different â€“ but the message is the same: when government acts to limit therapeutic choices, patients suffer the consequences.
Australia limits coverage of Alzheimerâ€™s medicines to six months of treatment unless the patient shows â€œsignificant improvement.â€ In the US, the decision to continue treatment is based on patient (and care-giver) satisfaction â€“ which includes maintenance of current mental status and prevention of mental decline â€“ quite a difference from â€œsignificant improvement.â€
Restrictive formularies (yes, like the VA system) and health care systems (such as in the United Kingdom, Australia, Canada, and elsewhere) that deny access to the right drug for the right patient at the right time but pay for more expensive and invasive procedures later on have their priorities upside down.