Swanson et al. (1997) found violent behaviour in psychiatric patients to be related to co-morbid substance abuse, the absence of recent contact with psychiatric services, and psychotic symptoms such as a feeling of being threatened and cognitive disorganization. In 96 adult schizophrenic patients from general psychiatry, greater numbers of misdemeanour convictions were linked to more severe drug and alcohol abuse histories and greater levels of disorganized symptoms, whereas a greater number of felony convictions was only associated with more severe drug abuse histories (Fukunaga and Lysaker 2013). Both the severity of severe drug abuse histories and levels of disorganized symptoms contributed to predicting 24 % of the variance in the number of reported lifetime misdemeanour offences. Soyka (2000) emphasized the importance of recurrent intoxication, so that the increased risk of aggression cannot be interpreted simply as the result of poor social integration. In a systematic review and meta-analysis, Fazel et al. (2009) identified 20 individual studies reporting data from 18,423 individuals with schizophrenia and other psychosis. Patients with schizophrenia and other psychosis were associated with violence and violent offending, particularly homicide. Co-morbidity with substance use disorders substantially increased the risk, with increased OR’s between 3 and 25. The increased risk of violence in these disorders with co-morbid substance abuse was not different than the risk of violence in individuals with diagnoses of substance use disorders. A recent systematic review and meta-regression analysis of 110 studies reporting on 45,533 individuals revealed that 18.5 % of whom were violent (Witt et al. 2013). A total of 39,995 (87.8 %) were diagnosed with schizophrenia, 209 (0.4 %) with bipolar disorder, and 5,329 (11.8 %) with other psychoses. Dynamic or modifiable risk factors included recent drug misuse, among others (p values < 0.0001), and higher impulse control scores, recent substance misuse, and recent alcohol misuse (p value < 0.01). In relation to premorbid factors, violence was moderately associated with parental history of alcohol misuse (OR = 1.8). Finally, Tengström et al. (2001) emphasized the importance of substance abuse in early starters (those schizophrenic patients with first conviction before the age of 18), due to both the presence of a diagnosis of substance abuse and the fact that most early starters were intoxicated at the time of the offence. Moreover, early starters differed from late starters in the prevalence of substance abuse by the parents, low grades at school, and a conduct disorder at an early age.

Our own study (Goethals et al. 2008) revealed that violent male psychotic offenders with a substance abuse-related disorder were significantly younger at the time of their first conviction, but they had not committed more violent, sexual offences or offences against property and had not spent more months in prison prior to the index offence than psychotic offenders without a co-morbid diagnosis of substance abuse. However, the prior criminal history was no more serious in those that were intoxicated at the time of the index offence than in those that were not intoxicated. We concluded that the role of substance abuse in psychotic offenders was related directly to the psychotic disorder and less to the criminal environment in which these patients find themselves. Recently, Kraanen et al. (2012) compared different types of offenders in forensic outpatient treatment, such as offenders of general violence, intimate partner violence, sex offences, and other offences such as drug trafficking and property crimes, regarding the prevalence of substance abuse disorders at the time of the offence. However, the principal diagnosis in all these offenders remained unclear. More general violence offenders and less sex offenders fulfilled diagnostic criteria for a substance use disorder. About 30 % of the offenders were intoxicated by substances at the moment they committed the offence. More general violence offenders were intoxicated during the offence. Finally, van Panhuis and Dingemans (2000) compared three Dutch cohorts of mainly male psychotic TBS detainees. This comparison also showed that the use of alcohol and drugs could aggravate violent behaviour in patients with psychosis.

In Finland, the likelihood of committing a violent offence was 25 times as high in male schizophrenic patients who used alcohol as in mentally health persons, compared to 3.6 times for patients with schizophrenia who did not use alcohol and 7.7 times for patients with other psychosis (Räsänen et al. 1998). In this study, patients with schizophrenia who did not use alcohol did not have relapses, in contrast to those who did use alcohol. In a New Zealand birth cohort, Arsenault et al. (2000) investigated the relation between mental illness and violence. Individuals with alcohol dependence, cannabis dependence, and a schizophrenic disorder had a 1.9, 3.8, and 2.5 times greater chance, respectively, of displaying violent behaviour. The individuals with at least one of these three disorders constituted one-fifth of the study population but were responsible for half of all violent offences. In persons with alcohol dependence, their violent behaviour could best be explained by the use of alcohol prior to the offence. In persons with cannabis dependence there was an association with a conduct disorder in childhood.

The assumption that substance abuse precedes violence in society was investigated by Cuffel et al. (1994). The chance of displaying violent behaviour was especially high in patients with a pattern of multiple drug use, including illegal drugs; Miles et al. (2003) reported that 34 % of their psychotic patients used alcohol, 22 % alcohol and cannabis, 12 % cannabis alone, and 24 % stimulants. A history of violent behaviour was seen significantly more often in the users of stimulants. There were hardly any other differences between the various subgroups of patients with various types of substance abuse. Corbett et al. (1998) found no indication that patients with schizophrenia prefer a particular type of drugs compared to patients with a personality disorder. Drug-abusing male inpatients with a personality disorder were significantly more likely than patients with schizophrenia to have consumed alcohol at the time of the violent offences. Case series of homicide offenders with schizophrenia show high levels of substance abuse co-morbidity (between 40 % and 71 %) according to Putkonen et al. (2004) and Bennett et al. (2011), which increases the odds ratio to 21 (Schanda et al. 2004). A survey based on a 3-year (1996–1999) consecutive sample of people convicted of homicide (n = 1,594) in England and Wales showed that more than one-third (42 %) occurred in people with a history of alcohol misuse or dependence and 40 % in people with a history of drug misuse of dependence (Shaw et al. 2006). Alcohol or drug misuse played a contributory role in two-fifths of homicides. Forty-two homicides (17 %) were committed by patients with severe mental illness and substance misuse. In the forensic outpatient sample of Kraanen et al. (2012) more general violence offenders and less other offenders were diagnosed with alcohol dependence, and more general violence dependence offenders were diagnosed with cannabis dependence at the time of the offence. Some authors have postulated that increasing substance use (particularly cannabis, cocaine, and amphetamines) was responsible for the increase of homicides committed by offenders with acute psychotic symptoms in England and Wales between 1997 and 2006 (Swinson et al. 2011).

What is the effect of substance abuse on the relation between violence and a psychotic disorder? The relationship between substance abuse and violence in psychotic disorders may be mediated by personality features and/or social problems, and is unlikely to be a simple additive effect (Mullen 2006). According to Smith and Hucker (1994), substance abuse was more prevalent among psychiatric patients than previously supposed. Schizophrenic patients, especially, were more susceptible to the negative effects of substance abuse, such as antisocial and violent behaviour. Philips (2000) arrived at a comparable conclusion: the prevalence of violent behaviour was higher in patients with both a psychiatric disorder and comorbid substance abuse than those with a single diagnosis. Such a dual diagnosis was a significant predictor of violent behaviour. Male schizophrenic patients in a large Finnish birth cohort were also found to be at high risk of committing a violent offence (Tiihonen et al. 1997). The prevalence of registered offences was highest among schizophrenic patients with co-morbid alcohol abuse and patients with an alcohol-induced psychosis. Steinert et al. (1996) compared a group of violent male schizophrenic patients with nonviolent schizophrenic patients; substance abuse was seen in 70 % of the aggressive male schizophrenic patients versus 13 % of the patients who had no history of violent behaviour. This is in agreement with the results of a study by Blanchard et al. (2000). According to them, substance abuse was seen in half of the violent schizophrenic patients, especially in young men.

A large retrospective study of hospitalized Swiss patients and a matched control group from the total Swiss population (Modestin and Ammann 1995) revealed that the number of criminal convictions was significantly higher among users of alcohol and drugs, independent of socio-demographic factors. The chance of a criminal record was twice as high among schizophrenic male patients with co-morbid substance abuse as in schizophrenic male patients without substance abuse (Modestin and Würmle 2005). In comparison with the rest of the population, however, the chance of having committed a violent offence was greater in schizophrenic patients without substance abuse.