A drug used to treat patients with Alzheimer's disease could actually make their condition worse, a study says.
There have also been concerns about other Alzheimer's drugs
Quetiapine (Seroquel) is commonly used in nursing homes to combat agitation, a common symptom of Alzheimer's.
But research by Institute of Psychiatry experts in the British Medical Journal online suggests it could significantly speed up the rate of patients' decline.
However, a spokesman for AstraZeneca, which makes Seroquel, said the drug was safe and effective.
Antipsychotic drugs such as quetiapine are used in up to 45% of nursing homes to treat agitation, which is a common and distressing symptom of dementia. They are also used to treat schizophrenia.
Ninety-three patients at care homes in the north-east of England who had Alzheimer's, dementia and significant levels of agitation were studied over six months.
They were split into three groups. One was given a daily dose of quetiapine, another was given the "anti-dementia" drug rivastigmine, and the third a dummy pill.
Researchers then assessed their agitation levels and cognitive abilities, such as memory skills, throughout the study.
Forty-six patients completed cognitive assessments after six weeks.
The 14 who were taking quetiapine registered an average drop of around 14 points on the scale used to assess decline, compared to almost no change for those taking the dummy pill.
Those who took rivastigmine showed little or no worsening of their illness - but no improvement in symptoms compared to the dummy pill group.
The study is of concern to Alzheimer's researchers as there have previously been worries about the safety of the two most commonly used antipsychotic drugs in people with dementia, risperidone and olanzapine, because of an increased risk of stroke.
But the researchers said their findings showed quetiapine should not be used instead of other drugs for alleviating their symptoms, and that they highlighted concerns over long term use of antipsychotics in Alzheimer's patients.
Dr Clive Ballard, who led the study, said: "Behavioural problems seen in people with dementia can be extremely distressing and present major difficulties for both patients and caregivers.
"This research shows that the drug quetiapine does not help with the agitation experienced by some patients and that it accelerates cognitive decline.
"We are extremely fortunate that people are willing to volunteer for this important research.
"Like us, they hope the project will lead to improved treatments for Alzheimer's and other causes of dementia."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, which funded the study, said: "These results show the huge and pressing need to develop new and safe treatments for people with dementia.
"Research into Alzheimer's is severely underfunded and we desperately need to do more to accelerate progress towards finding effective treatments, both for the symptoms and for the underlying disease."
However, a spokesman for AstraZeneca said: "The company remains confident in Seroquel's safety and efficacy profile, with more than eight million patients treated since its launch in 1997."
He said that the sample used was too small for conclusions about the drug's effect on cognitive decline to be made.
And he added: "One patient in the Seroquel arm of the study had an unusually large negative change in cognitive assessment.
"Two of the placebo [dummy pill] patients and two of the rivastigmine patients had unusually large improvements. These five patients disproportionately influenced the results of this small study."