In a 28-week study of Alzheimer's patients, 250 were in a randomized trial to memantine or placebo.

CNN: Better understanding of dementia leading to more effective therapies

Memantine is also especially useful in the treatment of patients with Alzheimer's disease and patients with vascular dementia.

CNN: Better understanding of dementia leading to more effective therapies

Compared with the group taking placebo, memantine-treated patients had less deterioration in mental function over the 28 weeks.

CNN: Better understanding of dementia leading to more effective therapies

Donepezil is one of a class of drugs known as cholinesterase inhibitors while memantine is a glutamate moderator.

CNN: Hard to tell, but drug helps as Alzheimer's progresses

Donepezil is approved in the U.S. for all stages of Alzheimer's while memantine is approved for moderate-to-severe Alzheimer's.

CNN: Hard to tell, but drug helps as Alzheimer's progresses

There are four licensed treatments for Alzheimer's - donepezil, galantamine, rivastigmine and memantine.

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Taking either memantine or donepezil alone was better than nothing at all, but taking them together offered no additional benefit.

CNN: Hard to tell, but drug helps as Alzheimer's progresses

Meanwhile, a fourth drug, memantine, which is used to treat moderate to severe Alzheimer's could also be withdrawn, Nice said.

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People who continued taking donepezil did better on two different scales of cognition and impairment than those taking no medication or those taking memantine alone.

CNN: Hard to tell, but drug helps as Alzheimer's progresses

Memantine should be started in patients with moderate to severe Alzheimer's.

CNN: Better understanding of dementia leading to more effective therapies

Dizziness is the most common side effect associated with memantine.

CNN: Better understanding of dementia leading to more effective therapies

Memantine has fewer side effects than the cholinesterase inhibitors.

CNN: Better understanding of dementia leading to more effective therapies

In April, a study appeared in The New England Journal of Medicine showing that patients on Aricept and memantine had a less steep decline on cognitive tests than those on Aricept alone.

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Investors should keep a close watch on the switch from Celexa to Lexapro and not worry too much about new indications for memantine, which is not yet approved for its primary indication, Alzheimer's.

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Donepezil was also more effective than another common Alzheimer's drug, memantine (Namenda), in patients with later-stage disease, although memantine still showed some benefit, according to the study published this week in The New England Journal of Medicine.

CNN: Hard to tell, but drug helps as Alzheimer's progresses

Memantine (Namenda) is another oral medication.

CNN: Better understanding of dementia leading to more effective therapies

Although the combination of donepezil and memantine -- a common strategy in the U.S., but not the U.K. -- had no added benefit, each drug had "independent positive effects, " says Howard, who advises patients at this stage of the disease to continue taking both drugs if possible.

CNN: Hard to tell, but drug helps as Alzheimer's progresses