Doctors are being warned not to routinely give people with learning disabilities anti-psychotic drugs to curb aggressive behaviour.
About 200,000 people with learning disabilities are treated with the drugs
An Imperial College London study of 86 patients found the drugs were no more effective than being given none at all.
Researchers said it was more important to address the underlying causes.
In the UK, 200,000 people with learning disabilities are given anti-psychotic drugs - even though there is a risk of side-effects, the Lancet reported.
These can include risk of weight gain, impotence and strain to the cardiovascular system.
The team studied patients in 10 inpatient and community settings in England, Wales and Australia.
One group was given haloperidol, a first-generation antipsychotic drug, a second group got risperidone, a second-generation version, while a third received a dummy pill.
Clinical assessments of aggression, aberrant behaviour, quality of life, adverse drug effects and feelings towards their carer were recorded at four, 12 and 26 weeks.
The researchers found that aggression had decreased substantially with all three treatments by week four, but patients receiving the dummy pill had the greatest change.
Improvements were seen with the other measures, but these were similar for all three groups.
Lead researcher Professor Peter Tyrer said: "The problem with patients with learning disabilities is that we haven't had the evidence on whether anti-psychotic drugs work.
"Therefore, these patients were assumed to be the same as other mental health patients.
"But what our research shows is that drugs are no better than not giving any drugs. It seems what is important is the care a person receives.
"When people with learning disabilities are aggressive it is important they are given support and people communicate with them."
But he added that there would still be exceptional circumstances where such drugs were necessary.
Dr Jim Kennedy, prescribing spokesman at the Royal College of GPs, agreed.
But he added: "All too often the drugs are used as a chemical restraint. This can be poor practice."
And David Congdon, from the Mencap charity, said: "Anti-psychotic drugs should be seen as a last resort.
"Challenging behaviour is caused by many different factors - an undiagnosed health condition causing extreme pain, frustration at not being able to communicate properly, or boredom due to a lack of meaningful activity.
"All of this can be dealt with without the use of anti-psychotic drugs."