Aggression is not an inevitable pharmacological consequence of misusing alcohol or any particular drug, but arises from many possible factors including expectancy effects, pattern of consumption, individual responses to intoxication or withdrawal, peer influences, and interpersonal issues.

Alcohol consumption is the single factor most associated with violence. It is a repeated finding from the British Crime Surveys that alcohol is a key factor in at least half of interpersonal assaults, with a greater contribution to assaults on strangers and domestic violence. By comparison, drug misusers are overwhelmingly more likely to commit acquisitive offences.

The co-morbidity of major mental illness and substance misuse further increases the risk of violence. For example, in the Epidemiological Catchment Area (ECA) survey of 10 000 individuals,⁽¹⁾ the prevalence of violent behaviour in the previous year was 2 per cent for those with no mental disorder, 7 per cent in ‘major mental illness’, 20 per cent for ‘substance misuse disorder’, and 22 per cent for co-morbid respondents. Among patients with first-episode psychoses,⁽²⁾ just under 10 per cent demonstrated serious aggression when psychotic and 23 per cent showed lesser degrees of aggression. Those co-morbid for drug misuse were nine times more likely to show aggression after service contact—primary drug-related psychoses or alcohol misuse were not so associated.

There is particular concern in the UK about the risk to the public from serious violence by the mentally disordered. Nationally,⁽³⁾ alcohol or drug misuse contributes to two-fifths of homicides and 17 per cent were committed by patients with severe mental illness and substance misuse. Alcohol- and drug-related homicides were generally associated with male perpetrators who had a history of violence, personality disorders, mental health service contact, and with stranger victims.

However, these epidemiological studies cannot define a causal link between substance misuse and mental disorder. There are many ways in which violence may arise in substance misusers and in co-morbid individuals. Simple intoxication on alcohol or other depressants such as benzodiazepines or barbiturates, leads initially to apparently excited behaviour. Stimulants such as cocaine or amphetamines, may produce arousal and irritability. Most forms of intoxication are also associated with impaired judgement, perception, and impulse control. Severe intoxication on alcohol, Cannabis, sedatives, or stimulants can lead to a toxic psychosis and highly disturbed behaviour. Even at levels of consumption insufficient to intoxicate, disinhibition, and autonomic arousal may facilitate recklessness and aggression. Pathological intoxication, in which aggression is supposed to occur within minutes of consuming moderate amounts of alcohol, is of doubtful validity and in most cases, better explained by alcohol-induced hypoglycaemia, head injury, or other organic disorder.

The association between withdrawal effects and potential for violence is often overlooked. Withdrawal from alcohol and most drugs of dependence, is a highly aversive state in which irritability and aggression may occur. Cessation from alcohol or sedatives, may lead to more severe withdrawal syndromes such as delirium tremens which are commonly associated with impaired perception, affect, judgement, and impulse control.

The relationship between acquisitive crime and drug misuse problems was studied among 753 clients recruited to the National Treatment Outcome Research Study (NTORS).⁽⁴⁾ More than 17 000 offences were reported during the 90-day period prior to treatment. Half of the clients committed no acquisitive crimes during this period, whereas 10 per cent committed 76 per cent of the crimes.

Such work does not demonstrate a causal relationship between illicit drug use and acquisitive crime. From a large survey of British youth,⁽⁵⁾ the average age of onset for truancy and crime are 13.8 and 14.5 years respectively, compared with 16.2 for drugs generally and 19.9 years for ‘hard’ drugs. Thus, crime tends to precede drug use rather than vice versa. It is clear that heavy drug use is strongly associated with impulsive acquisitive offending, including street robbery, and burglary, which involve violence.

In Britain, the non-medical use of drugs is subject to the Misuse of Drugs Act 1971, as subsequently amended, and which contains a classification based on perceived harm. Class A drugs include Ecstasy, LSD, heroin, cocaine, crack, magic mushrooms (if prepared for use), amphetamines (if prepared for injection); Class B drugs include amphetamines and methylphenidate (ritalin); Class C drugs include Cannabis, tranquilizers, some painkillers, Gamma hydroxybutyrate (GHB), ketamine. In January 2004, Cannabis was reclassified from a Class B to a Class C drug, it is still illegal. This legislation defines the penalties for supply, dealing, production, trafficking, and also possession.

Other offences include driving cars, or public conveyances such as trains whilst under the influence of alcohol or other drugs.