Many dementia patients being prescribed "chemical cosh" antipsychotic drugs could be better treated with simple painkillers, research says.The British and Norwegian study, published on the BMJ website, found painkillers significantly cut agitation in dementia patients.
Agitation, a common dementia symptom, is often treated with antipsychotic drugs, which have risky side effects.
The Alzheimer's Society wants doctors to consider other types of treatment.
Experts say that each year about 150,000 patients in the UK are unnecessarily prescribed antipsychotics, which have a powerful sedative effect, and can worsen dementia symptoms, and increase the risk of stroke or even death.
They are often given to patients whose dementia makes them aggressive or agitated.
But researchers from Kings College, London, and Norway speculated that the behaviour may sometimes be caused by pain, which patients were unable to express in other ways.
They studied 352 patients with moderate or severe dementia in nursing homes in Norway.
Half were given painkillers with every meal, the rest continued with their usual treatments.
Supervised treatment 'key'
After eight weeks, there was a 17% reduction in agitation symptoms in the group being given painkillers - a greater improvement than would have been expected from treatment with antipsychotics.
The researchers concluded that if patient's pain was properly managed, doctors could reduce the number of prescriptions for antipsychotic drugs.
Professor Clive Ballard, one of the report authors and director of research at the Alzheimer's Society, said the finding was significant.
"At the moment, pain is very under-treated in people with dementia, because it's very hard to recognise," he said.
"I think this could make a substantial difference to people's lives - it could help them live much better with dementia."
However, he said painkillers should only be given to patients under the supervision of a doctor.
The Alzheimer's Society is issuing new guidance calling on doctors to think much harder before prescribing antipsychotics, and to look at prescribing pain medication instead.
The National Care Association said the study highlighted some of the complexities of dementia.
"Pain in itself is debilitating, so to identify it as the route cause of agitation and aggressive behaviour is a major breakthrough which will enable us to support people appropriately," said its chairman, Nadra Ahmed.
A government programme to reduce the inappropriate prescription of antipsychotic drugs is already under way in England.
The care services minister Paul Burstow welcomed the study.
"It should act as a further call for GPs to carefully examine the reason why those with dementia display agitated behaviour, rather than immediately resorting to antipsychotic medication," he said.