Neuroleptics the drugs given to treat dementia, including Alzheimer's can make the condition worse.
Patients who took the drugs found that their cognitive abilities knowing, thinking, learning and judging declined twice as quickly as those never given them.
This decline coincided almost exactly with the start of the drug therapy, researchers from Warneford Hospital in Oxford discovered.
They studied 71 people suffering from dementia, 16 of whom were taking neuroleptics. As well as suffering a more rapid decline of their cognitive abilities, they also became more aggressive, suffered sleeping problems and became paranoid.
The drugs have already been banned from nursing homes in the United States to treat behaviour such as wandering, restlessness, insomnia and agitation, while a recent study in Glasgow suggested that up to 88 per cent of patients in nursing homes should not be given them.