A US study of the antidepressant citalopram has found severe side effects when given to patients with Alzheimer's disease.
The team at the University of Rochester in New York reported the drug can reduce agitation, but can also worsen the condition and raise the chances of heart complications.
Before 2009, antipsychotic medications were commonly given to people with Alzheimer's due to their ability to lessen agitation and distress.
However, a study funded by the charity Alzheimer's Research UK found that long-term usage could double the risk of death.
This latest trial backs up the charity's findings.
Some 186 people with Alzheimer's were recruited, all of whom suffered from regular agitation. For a period of nine weeks, half of the group were given 30 milligrams of citalopram, while the rest of the group received a placebo.
In those given the drug, 40 per cent showed signs of reduced agitation compared to 26 per cent of people given the dummy pill.
However, tests found that the first group showed a slight decline in thinking skills and memory as well as changes in the electrical activity of their hearts.
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "Although this small study indicates that citalopram may help with symptoms of agitation in Alzheimer's, the adverse side effects recorded suggest that at the doses used in the trial, this drug may not be safe for people with the disease.
"Further, larger trials would be needed to determine whether citalopram could be used safely to treat agitation in lower doses."
He added that the study highlights the need for safer alternatives to be developed.
According to the Alzheimer's Society, there are currently around 820,000 people living with some form of dementia in the UK and the figure is likely to rise above the one million mark unless a significant breakthrough in research is made.