The study of Aricept also found some benefit if the patient switches to another frequently used drug, Namenda. But it found that combining the two drugs, a common strategy in the United States, was not significantly more effective than using Aricept alone.
That last finding conflicts with an earlier study, and experts said it would probably not change what most doctors do in the United States.
The overall study results generally provide statistical support for what most doctors recommend, said Dr. Ronald Petersen of the Mayo Clinic, a member of the Alzheimer's Association board of directors. He didn't participate in the research.
More than five million Americans have Alzheimer's, according to the association. The progressive disorder impairs memory, judgment and other mental abilities. Treatments can only ease symptoms rather than reverse a patient's overall decline.
The new research is reported in Thursday's issue of the New England Journal of Medicine by scientists in the United Kingdom. It involved 295 patients who had moderate or severe Alzheimer's and had been taking Aricept, also known as donepezil, for at least three months.
The patients were randomly assigned to continue on Aricept, switch to Namenda, take both, or take only placebo pills. Then they were followed for a year and tested on mental prowess and ability to carry out daily activities.
Results showed that continuing on Aricept gave a modest benefit over taking no drug, while switching to Namenda, also called memantine, provided a lesser one. While an analysis found no evidence that combining the drugs gave a significant benefit over just continuing Aricept, it may have included too few patients to demonstrate a difference, said Dr. Robert Howard of the Institute of Psychiatry at King's College London, who led the research.
A separate analysis indicated each drug provided an independent benefit, Howard said, so he still recommends combining them if possible in moderate to severe Alzheimer's.
The work was financed by the U.K. Medical Research Council and Britain's Alzheimer's Society. Drugs were supplied by Pfizer Inc., Eisa Inc. and Lundbeck UK. Several authors reported having received lecture fees or other payments from those companies.
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