Thousands of people with learning disabilities are being prescribed anti-psychotic drugs despite little evidence that they work, experts have warned.
The drugs are being used to tackle challenging behaviour in people who do not have a record of severe mental illness.
Other types of medicines to treat mental illness - such as anxiety drugs and anti-depressants - are also being prescribed in large numbers.
Experts from University College London (UCL) analysed data from more than 33,000 people with a learning disability, now sometimes called intellectual disability.
This includes people with Down's syndrome, autism and some people with epilepsy. They may have difficulties with learning, communication, daily living skills, information processing and social skills.
The study, published in the British Medical Journal (BMJ), analysed the GP records of 33,016 UK adults with intellectual disabilities, from between 1999 and 2013.
It found that 9,135 had been prescribed anti-psychotic drugs, of whom 71% had no record of severe mental illness.
Of the 11,915 people with a record of challenging behaviour, 47% had received anti-psychotic drugs, whereas only 13% had a record of severe mental illness.
Meanwhile, 26% of those given anti-psychotics did not have a record of severe mental illness or challenging behaviour.
The authors concluded that people with a record of challenging behaviour were more than twice as likely to receive anti-psychotics as those without such a history.
Prescriptions for anti-psychotics were also significantly more common in older people and those with depression, anxiety, autism, dementia and epilepsy.
Dr Rory Sheehan, from UCL's department of psychiatry, who worked on the study, said: "The number of people with intellectual disabilities who have been prescribed anti-psychotics is greatly disproportionate to the number diagnosed with severe mental illness for which they are indicated."
Anti-psychotic drugs are designed for severe mental illnesses such as schizophrenia but there is little evidence that they help address behavioural problems, the experts said.
Although the prescription of anti-psychotic drugs has declined over the past 15 years, prescribing still goes against good medical practice.
Dr Sheehan said: "Research evidence does not support using anti-psychotics to manage behaviour problems in people with intellectual disabilities.
"Many people with intellectual disability and behaviour disturbance have complex needs and other interventions, such as looking at the support people receive and their communication needs, should be prioritised.
"Anti-psychotics, or indeed any medications, should not be prescribed lightly and are no substitute for comprehensive care."
Dr Sheehan said some GPs may prescribe the drugs due to a lack of readily-available alternatives or as a short-term measure for people experiencing a crisis.
Dan Scorer, head of policy at mental health charity Mencap, said: "Sadly, the report findings are not a surprise as they confirm what we have heard from families time and time again about their loved ones being given high levels of anti-psychotic or anti-depressant medication, often for years.
"In many cases families report serious side-effects and no evidence that the medication is helping the individual."
Viv Cooper, chief executive at the Challenging Behaviour Foundation, said: "Individuals who display behaviours which challenge are at increased risk of being inappropriately prescribed powerful drugs meant for the treatment of an illness they do not have.
"This is not a recognised or humane response to the needs of vulnerable individuals."