May 19 (HealthDay News) -- Substance abuse significantly increases the likelihood that people with schizophrenia will commit a violent crime, new research says.

Researchers in England examined data on more than 8,000 people with schizophrenia, a serious and disabling mental illness that causes delusions, hallucinations and paranoia. The patients were included in a nationwide Swedish registry of hospital admissions and criminal convictions from 1973 to 2006.

The investigators found that patients with schizophrenia were twice as likely to have committed a violent offense than the general population, according to the study in the May 20 issue of the Journal of the American Medical Association.

Some 13.2 percent of the patients with schizophrenia had at least one violent offense compared with 5.3 percent of the general population, the study authors reported.

But, the findings also show that the violent crime rate for those patients who did not abuse drugs or alcohol was much lower and only a few percentage points higher than the general population.

Nearly 28 percent of patients diagnosed with schizophrenia and substance abuse had committed a violent crime, while only 8.5 percent of schizophrenics who didn't abuse drugs or alcohol had committed a violent offense, according to the study authors.

Many studies have reported on the association between major mental disorder and violence, including some that specifically have examined the relationship with schizophrenia, the researchers said.

Most studies have found a fourfold to sixfold increase in the risk of violent behavior, but the reasons for the tendency toward violence is unclear.

"We demonstrate that the risk of violent crime in schizophrenia in patients without comorbid substance abuse is only slightly increased," the study authors wrote. "In contrast, the risk is substantially increased among patients with comorbidity and suggests that current practice for violence risk assessment and management in schizophrenia may need review."

Familial factors, either genetic or environmental, may also play a role in who is likely to commit a violent crime, the researchers noted.

SOURCE: Journal of the American Medical Association, news release, May 19, 2009