Anti-psychotic drugs commonly given to Alzheimer's patients often make their condition worse, a UK study suggests.
Neuroleptics provided no benefit for patients with mild behavioural problems, but were associated with a marked deterioration in verbal skills.
The research focused on 165 people with advanced Alzheimer's who were living in nursing homes in four British cities.
Up to 60 per cent of Alzheimer's patients in nursing homes are given the drugs to control behaviour such as aggression.
The study appears in the journal Public Libary of Science Medicine.
The researchers, from Kings College London and the Universities of Oxford and Newcastle, found the drugs offered no long-term benefit for most patients with mild symptoms of disturbed behaviour.
But just six months of treatment was enough for patients to show a marked deterioration in their verbal fluency.
Further preliminary analysis already under way on the data suggests the use of neuroleptics may also increase death rates.
The research focused on patients living in nursing homes in Oxfordshire, Newcastle, Edinburgh and London.
All patients had been taking neuroleptics for three months. They either continued on the same medication for a further 12 months, or took a dummy pill.
Lead researcher Professor Clive Ballard, said: "It is very clear that even over a six-month period of treatment, there is no benefit from neuroleptics in treating the behaviour in people with Alzheimer's disease when the symptoms are mild.
"For people with more severe behavioural symptoms, balancing the potential benefits against adverse effects is more difficult."
Rebecca Wood, of the Alzheimer's Research Trust, said: "These results are deeply troubling and highlight the urgent need to develop better treatments."
The trust says that neuroleptics should only continue to be prescribed long-term to dementia patients with severe behavioural problems, and then only as a last resort when non-drug methods have been tried and have failed.
Neil Hunt, of the Alzheimer's Society, said previous research had also shown that anti-psychotic drugs raised the risk of stroke and death for people with dementia.
"This widespread overprescription to people with dementia must stop," he said.
"It is time we stop wasting money giving people drug treatments with no benefit and start investing in good quality dementia care."
A report into the use of anti-psychotics in care homes is due to be published by the All-Party Parliamentary Group on Dementia this month.
The neuroleptics which came under analysis in the study were thioridazine (Melleril), chlorpromazine (Largactil), haloperidol (Serenace), trifluoperazine (Stelazine) and risperidone (Risperdal).