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AIMS:

To investigate the triggering effects of alcohol, illegal substances and major classes of prescribed psychotropic drugs on criminal violence.

DESIGN:

A case-crossover design, using each subject as its own control.

SETTING AND PARTICIPANTS:

A consecutive sample of 133 violent offenders was recruited from a forensic psychiatric evaluation unit and a national prison evaluation unit in Sweden during 2002-03.

MEASUREMENTS:

Offenders were assessed with structured interviews. Risk estimates were based on hazard periods of 24 hours. We used standard Mantel-Haenszel methods for statistical analyses.

FINDINGS:

A 13.2-fold increase of risk of violence [95% confidence interval (CI): 8.2-21.2] was found within 24 hours of alcohol consumption. This increase in violence risk was similar among individuals combining alcohol with benzodiazepines [Relative risk (RR) = 13.2, 95% CI: 4.9-35.3]. Use of benzodiazepines alone in regular doses (RR = 0.4, 95% CI: 0.2-0.5) or antidepressants [selective serotonin reuptake inhibitors (SSRIs) or tricyclics] (RR = 0.4, 95% CI: 0.3-0.8) was associated with a lowered risk for violence.

CONCLUSIONS:

This study confirmed that alcohol is a strong trigger of criminal violence. Benzodiazepines in combination with alcohol caused no further increase of violence risk. Benzodiazepines in regular doses and antidepressants may inhibit violence, but further studies are needed to verify causality. The case-crossover method can contribute to research on the proximal causes of criminal violence.