Antipsychotic drugs which are commonly used to treat aggression in people who are intellectually disabled have been revealed to be ineffective.
The researchers say such candidates should not be prescribed these drugs as placebos seem to do a better job.
A study by researchers at Imperial College, London has found that when two antipsychotic drugs haloperidol and risperidone were compared with a placebo, the placebo reduced aggression by nearly 80 percent compared with 50 percent to 60 percent for the drugs.
In order to gauge aggression the researchers used a widely accepted measuring scale.
The research was conducted on 86 non-psychotic patients with aggressively challenging behaviour at 10 centers in England and Wales and one in Australia.
All but one of the patients was in a community setting, more than half were men, and the majority had mild or moderate intellectual disability.
The researchers say at no point was the placebo significantly worse than risperidone or haloperidol, nor was there a difference in response for the small group of patients with autism spectrum symptoms.
Despite little evidence to support their use, antipsychotic drugs are prescribed from 22% to 45% of inpatients in this population and to about 20% of those in the community.
Risperidone is sold under the brand name Risperdal and is a popular schizophrenia drug, Haloperidol is an older drug marketed as Haldol and both are manufactured by drug company Johnson & Johnson.
According to Dr. Peter Tyrer, a community psychiatrist, who led the study, such drugs have been used to treat aggression in people who are not psychotic for 60 years.
Dr. Tyrer says in most circumstances people should not be given these drugs because there is no indication they work; he says their study shows that a placebo is more effective.