LONDON (Reuters) - Pfizer's dementia drug Aricept, already commonly used to treat mild to moderate Alzheimer's disease, can also help patients with severe disease and should be used more widely and for longer, according to research published on Wednesday.

British scientists who studied the possible longer-term benefits of giving Aricept suggested that extending treatment could help twice as many Alzheimer's sufferers worldwide.

The study also looked at another commonly used dementia drug called memantine, which is sold in the United States under the brand Namenda by Forest Laboratories and Germany's Merz, and in Britain under the brand Ebixa by Danish group Lundbeck.

It found that keeping patients with moderate to severe Alzheimer's on Aricept, or donepezil as it is known generically, or starting them on memantine treatment, meant they had significantly better cognitive and function abilities than patients taking a placebo or dummy pill.

An estimated 18 million people worldwide suffer from Alzheimer's disease, which is the most common form of dementia. It is fatal brain disease that affects memory, thinking, behaviour and the ability to handle daily activities and is placing an increasingly heavy burden on societies and economies across the world.

According to the World Health Organisation, some 35 million people worldwide have dementia, and Alzheimer's Disease International predicts that as populations age, dementia cases will almost double every 20 years to around 66 million in 2030 and 115 million in 2050.

The study, published in the New England Journal of Medicine, involved 295 Alzheimer's patients in Britain who were assigned to one of four separate groups - one continuing to take donepezil, one stopping donepezil and getting a placebo, one stopping donepezil and starting memantine, and a fourth taking both drugs together.

Robert Howard, a professor at King's College London who led the trial, said it was the first to show the value of continued drug treatment for patients with moderate to severe Alzheimer's.

While donepezil is commonly prescribed for patients in the early stages of the disease, doctors in some countries, including Britain, are advised to stop prescribing the drug to patients once their disease has progressed to become more severe.

"As patients progress to more severe forms of Alzheimer's disease, clinicians are faced with a difficult decision as to whether to continue or not with dementia drugs and, until now, there has been little evidence to guide that decision," Howard told reporters at a briefing abut his findings.

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Study finds key dementia drugs should be used more