Safety watchdogs will "carefully consider" research suggesting a drug used to curb aggression and agitation in thousands of people with dementia hastens their mental decline.

Quetiapine, marketed as Seroquel, is taken by up to 45% of those with Alzheimer's disease and similar conditions in residential and nursing homes.

It is often prescribed for long periods to treat distressing behavioural symptoms in patients and it has been thought safer than other drugs. Studies of people in institutional care in Newcastle, led by Clive Ballard, of King's College London, suggest the use of quetiapine worsens their illness. "These results have clear implications for clinical practice", he said in today's medical journal BMJ Online First.

The drug should not be used as an alternative treatment, he said, after trials involving 80 patients, one group on quetiapine, a second on another drug rivastigmine, and a third on dummy treatment. Neither of the two drugs were effective in treating agitation while quetiapine was associated with significantly greater cognitive decline.

AstraZeneca, manufacturer of Seroquel, said it was confident about the safety and efficacy of the drug, which had been used by 8 million people. It said the results were based on "a very small sample" and a handful of unusual results among patients in the trials had disproprtionately influenced results.

The maximum Seroquel dose used had been too low and there was no information on other medications patients were taking.

The Department of Health said its medicines and healthcare products regulatory agency "will carefully consider this new research".