Treating substance abuse issues in a person with severe mental illness will reduce the risk they will commit violent acts, a new study suggests. Health professionals have disagreed about whether to treat substance abuse or mental illness first in people who are dealing with both.

While most people with mental illness are not violent, those who have severe mental illness are more likely than those in the general population to commit violent acts, HealthDay reports.

“We were surprised to find that the severity of the patient’s psychiatric symptoms was not the primary factor in predicting later aggression. Rather, the patient’s substance abuse was the factor most closely associated with future aggression,” study co-author Clara Bradizza of the University at Buffalo Research Institute on Addictions said in a university news release.

The study included 278 patients, who were followed for six months after enrolling in an outpatient treatment program for substance abuse and mental illness.

“Our findings suggest that treatment attendance is very important for these individuals and treatment programs should include interventions that are likely to decrease substance abuse, as this may provide the additional benefit of reducing the risk of later aggression among dual-diagnosis patients,” Bradizza said.

The findings appear in the Journal of Substance Abuse Treatment.