People with mental illness are no more likely to commit violent crimes than ordinary members of the public – unless they have abused drink or drugs, researchers say.

Substance abuse is the chief cause of violent crime and increases the risk equally in people with and without mental illness, researchers at the University of Oxford found.

Mental illness is linked with a propensity to abuse drink and drugs – possibly to ameliorate the unpleasant side-effects of powerful psychiatric medicines – which in turn increases the risk of violence.

Substance abuse increased the risk of violence by between six and seven times among both the mentally ill and the general public, the researchers say. But it was much more common among the mentally ill, affecting between a fifth and a quarter, compared with a fiftieth of the public.

Dr Seena Fazel, a forensic psychiatrist at the University of Oxford, said: "Most of the relationship between violent crime and serious mental illness can be explained by alcohol and substance abuse. It's probably more dangerous walking outside a pub on a late night than walking outside a hospital where patients have been released."

The findings are based on a study in Sweden which has one of the most comprehensive registers of people with mental illness in the world and has similar characteristics to the UK. It is published in the journal Archives of General Psychiatry.

In England and Wales there are about 55 homicides a year committed by mentally ill people and more than a third of the population associate mental illness with violence.