Determining the aetiology of co-morbidity results in a chicken and egg discussion: what came first? Existing research about the causal relations between psychiatric and substance disorders is inconclusive. The symptoms of mental disorder and addiction problems interact and mutually influence each other.

Research evidence indicates that psychiatric and personality disorders usually occur before substance use disorders, i.e. they increase individuals’ susceptibility to such problems (e.g. Kessler et al., 2001; Bakken et al., 2003); however, psychiatric disorders may also be aggravated by drug use (e.g. for depression: McIntosh and Ritson, 2001) or occur in parallel.

Drug use can also be perceived as a component or symptom of a psychiatric or personality disorder and as an attempt to self-medicate (e.g. Williams et al., 1990; Murray et al., 2003). The fact that substance use alleviates distressing symptoms encourages the development of addiction. Once the drug use is discontinued, e.g. through withdrawal or substitution treatment, the symptoms may reappear. Acute drug-induced psychoses occur particularly in users of cocaine, amphetamines and hallucinogens and usually subside relatively quickly. It may, however, be very difficult to differentiate between symptoms due to substance intoxication and unrelated psychotic episodes.

Krausz (1996) suggests four categories of dual diagnosis:

Similarly, the Swedish national report differentiates between psychiatric patients suffering from co-morbidity and drug-dependent clients with personality disorders that are aggravated by drug use and which are not always adequately diagnosed.

Morel (1999) distinguishes non-specific psychiatric disorders found among drug addicts from complications specifically connected with drug use. Disorders often found among drug users include:

Problems specifically linked to drug use include:

Recent neuropsychological and neurobiological studies and the advent of techniques that enable brain processes to be visualised have made it possible to hypothesise about the interactions between mental and physical trauma, brain development, drug effects, stress and mental development. The reward system is essential in the development of addiction, and addiction is linked to structural changes and adaptation of the brain at micro and macro levels (Nestler, 2001).

Other theories link specific drugs to specific mental disorders, e.g. it has been postulated that heroin could reduce stress, alleviate pain and abolish menacing voices in schizophrenic and borderline schizophrenic patients; however, patients with severe mental illnesses do not use heroin. Cocaine, it is suggested, could lighten depressive states, allow behavioural disinhibition and permit narcissistic personalities to act out grandiosity. Cannabis could relieve tension and ecstasy ease social inhibitions (Verheul, 2001; Berthel, 2003).

The 2002 Irish national report specifically examined depression among drug users, based on the results of several studies, and concluded that there is a strong association between problem drug use, in particular use of opiates and benzodiazepines, and high rates of depression. German research into the relationship between substance disorders, depression and suicide concluded that the risk of suicide is greatly increased in those with a depressive disorder (Bronisch and Wittchen, 1998). It is possible that some individuals with depression may self-medicate with opiates and benzodiazepines: clients in treatment programmes have lower depression scores than clients in low-threshold services (Rooney et al., 1999) or at initiation of treatment (McIntosh and Ritson, 2001).

A survey in Norway (n = 2 359) found that a high proportion of drug users have experienced severe family problems during childhood and youth. Seventy per cent had experienced learning and behavioural problems in school, 38 % had been the victim of bullying and 21 % had received psychiatric treatment during childhood and adolescence (Lauritzen et al., 1997). Female drug users with psychiatric co-morbidity have often been the victims of traumatic sexual abuse (e.g. Beutel, 1999).