A controversial drug used to treat mild to moderate Alzheimer's disease can also help in the later stages of the disease, Swedish research suggests.
Drug is currently under consideration
A Karolinska Institute study of 194 patients published in the Lancet found donepezil could improve mental function in severe cases.
The drug is not licensed in the UK to treat severe Alzheimer's and the NHS may soon stop funding it in mild cases.
Campaigners say it should be available to those with all forms of Alzheimer's.
Of those suffering from Alzheimer's, some 20% are in the later and most severe stages of the disease.
As their health deteriorates, patients become less able to communicate, less mobile, and increasingly reliant on nursing care.
The researchers found patients given donepezil - known commercially as Aricept - showed an improved ability to think, remember and interact socially.
They were also more able to carry out daily activities compared with those on a dummy drug.
More patients on the active drug had side effects than those in the placebo group - but these were usually transient and mild, the team said.
Lead researcher Professor Bengt Winblad said: "Donepezil slows and can reverse some aspects of deterioration of cognition and function in individuals with severe Alzheimer's who live in nursing homes.
"If treatment can help patients in the late phase of dementia, without necessarily increasing the length of time they have severe Alzheimer's disease, then this is a treatment option that should be available."
Donepezil is one of a class of drugs, called cholinesterase inhibitors, which work by increasing the level of a brain chemical called acetylcholine.
It is currently licensed for patients with mild and moderate Alzheimer's but is being reviewed by drugs watchdog the National Institute for Health and Clinical Excellence (Nice).
Draft guidelines published in January recommend the drug should only be funded by the NHS for patients who reach a moderate stage of the condition.
Campaigners are angry that if these are accepted thousands of patients would not be entitled to the treatments in the early stages of Alzheimer's.
Rebecca Wood, of the Alzheimer's Research Trust, said other studies had also suggested that donepezil might help people with advanced Alzheimer's.
She said: "This and other recent research throws doubt on the conventional wisdom that cholinesterase inhibitors are only suitable for patients in the mild to moderate stages of the disease.
"It also suggests that these drugs may work in more ways than one and have other effects than simply inhibiting cholinesterases."
Ms Wood said only one other anti-Alzheimer's drug, memantine, had been shown to benefit patients in the later stages of the disease.
Susanne Sorensen, head of research at the Alzheimer's Society, said donepezil was only likely to produce small changes in people in the more advanced stages of the disease.
But she added: "These small changes can make an enormous difference to a person's life. It can mean being able to indicate when you are hungry or thirsty or increase a person's mobility, preventing painful bedsores."
She said the study emphasised the need to make the drug available to people with all stages of Alzheimer's.