Patients with dementia are dying early because they are being prescribed sedative drugs inappropriately in nursing homes, warn researchers.
The drugs were designed to treat schizophrenia
A five-year investigation revealed the antipsychotic drugs were being used as a 'chemical cosh' to control patients, contrary to expert advice.
Patients prescribed these drugs were dying on average six months earlier, the Alzheimer's Research Trust found.
But GPs said the drugs were only used "as a last resort".
Guidelines say they can be given if the patient is severely agitated or violent.
But lead researcher Professor Clive Ballard says in the majority of cases the prescriptions are inappropriate and do more harm than good - doubling the risk of early death.
Estimates suggest that as many as 40% of nursing home residents with Alzheimer's disease - 150,000 people - are prescribed these drugs, known as neuroleptics.
Medicines safety experts know that patients with dementia are three times more likely to have a stroke if they take the drugs.
Professor Ballard and his colleagues from King's College London studied 165 residents with Alzheimer's disease at more than 100 UK nursing homes who were on a neuroleptic drug - chlorpromazine, haloperidol, risperidone, thioridazine or trifluoperazine.
They stopped the drugs for half of the patients and switched them onto dummy pills instead for 12 months.
The differences in survival between the two groups were striking.
At 24 months, 78% of the dummy pill group were alive compared with 55% of those still on the neuroleptics.
At 36 months the figures were 62% versus 35%, and at 42 months 60% versus 25%.
Professor Clive Ballard said: "It is very clear that even over a six month period of treatment, there is no benefit of neuroleptics in treating the behaviour in people with Alzheimer's disease when the symptoms are mild."
Instead, psychological therapies should be used, he told the charity's conference in Edinburgh.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "These results are deeply troubling and highlight the urgent need to develop better treatments.
"Seven hundred thousand people are affected by dementia in the UK, a figure that will double in the next 30 years. The government needs to make Alzheimer's research funding a priority."
Neil Hunt, chief executive of the Alzheimer's Society, said: "It is a disturbing revelation that confirms some of our worst fears about neuroleptics, which have been the subject of numerous health warnings.
"It is a national scandal that people are being sedated in this way."
He said training for care staff must now be an urgent priority.
Professor Mayur Lakhani, chair of the Royal College of General Practitioners, said: "We would like to reassure patients, relatives and carers that neuroleptic drugs are not routinely prescribed to patients with dementia, and are used only as a last resort when patients suffer from severe episodes."
But he agreed that there needed to be vigilance about the prescribing of these drugs for dementia.
Sheila Scott of the National Care Association said: "There are some serious issues here that care homes, doctors and the Department of Health need to look at."
Gordon Lishman of Age Concern said: "Alzheimer's can be a very difficult condition for patients and those around them but the over reliance on sedatives is not the answer."
The Medicines and Healthcare products Regulatory Agency said it would continue to monitor the unlicensed use of neuroleptics in the treatment of patients with Alzheimer's disease and would carefully review the new study to see what further action might be necessary.