LONDON (Reuters) - Alzheimer's patients prescribed antipyschotic drugs as sedatives are dying early because of the treatment, British researchers said on Friday.

Although so-called neuroleptic drugs were originally developed for schizophrenia, they are frequently also used on an "off-label" basis to calm difficult or aggressive dementia patients.

A five-year investigation found that the drugs, when given to Alzheimer's sufferers, were linked to a significant increase in long-term mortality -- with patients on the medicines dying an average six months earlier than those given placebo.

Researchers led by Professor Clive Ballard of King's College London also found neuroleptics were associated with a significant deterioration in verbal fluency and cognitive function.

Ballard, who has criticized the use of such drugs in dementia patients in the past, said the latest study showed there was no benefit in giving neuroleptics to people with mild Alzheimer's.

For people with more severe behavioral problems, doctors had to balance potential benefits against the increased mortality, he added.

Up to 45 percent of people with Alzheimer's in nursing homes are prescribed neuroleptics as sedatives, according to the Alzheimer's Research Trust, which funded the study.

Ballard's colleague Professor Robin Jacoby of Oxford University said the causative link between neuroleptics and early death was unclear but past studies had implicated the drugs in adverse cerebrovascular events, such as mini-strokes.

"We don't know what the mechanism is and we need to explore it further," Jacoby said.

The study involved 165 patients with Alzheimer's who were analyzed between 2001 and 2006. The findings will be submitted for publication in the New England Journal of Medicine.

Drugs used in the clinical trial included Johnson & Johnson's Risperdal and four older types of antipsychotics.