TUESDAY, Jan. 10 (HealthDay News) -- A drug commonly used to treat patients with Alzheimer's disease does not appear to be effective for people older than 40 years who have Down syndrome and Alzheimer's, according to a new study.

Although previous animal studies of the Alzheimer's drug, memantine, showed promising results in mice with Down syndrome, this new study of people with Down syndrome aged 40 and older revealed the opposite, the researchers reported in the Jan. 9 online edition of The Lancet.

Memantine was given to 88 people with Down syndrome for one year, while another 85 patients received a placebo (the "control" group). Some of the participants had Alzheimer's and some didn't.

The investigators found that the brain function of the people in both groups declined equally.

Serious adverse effects were experienced by 11 percent of the group that took the medication. Meanwhile, 7 percent of the placebo group had similar adverse events. Five people from the medication group died because of these events, compared to four in the control group.

"Memantine is not an effective treatment in this group of patients. We believe that this robust finding will have implications for clinical practice and research strategy in the future. Specifically, therapies that are beneficial for people with Alzheimer's disease are not necessarily effective for the treatment of cognitive impairment or dementia in the context of Down syndrome," the study's author, Clive Ballard, a professor at the Wolfson Centre for Age-Related Diseases at King's College London, said in a journal news release.

Because nearly 40 percent of people with Down syndrome over 60 years of age are diagnosed with dementia, the study authors pointed out that more research is needed to determine the best way to treat dementia in these individuals.

"Further investment is urgently needed to develop treatments that are effective in this important group of people," the study's co-author, Anne Corbett, research manager at Alzheimer's Society (U.K.), stated in the news release.

More information

The U.S. National Institute of Neurological Disorders and Stroke provides more information on Alzheimer's disease.