The role of substance abuse in HIV infection is well documented. According to a report of the US SAMHSA, drug abuse behavior plays the single largest role in the spread of HIV infection in the United States today. Half of all new HIV infections now occur among injection drug users (Department of Health and Human Services 2008).

Although the prevalence of HIV positivity in people with SMI varies substantially (1.3–23.9 %), it is much higher than the HIV prevalence rate found in the general population (De Hert et al. 2009a, 2011a). Next to injection drug use, substance abuse-associated sexual risk behaviors, as well as a reduced knowledge about HIV-related issues, contribute to these higher HIV prevalence rates (Himelhoch et al. 2007; De Hert et al. 2011a, b). Meade (2006), for example, found that among persons with dual disorders, active substance abusers engaged in the highest rates of sexual activity (56 %), followed by persons with remitted SUD (46 %), and, finally, by those with no lifetime history of SUD (23 %). SMI persons with lifetime SUD were more than 14 times more likely than persons with no SUD to report partner-related risks, including multiple partners, non-monogamous partners, sex with prostitutes or strangers, and sex trade. Individuals with SMI who have a history of childhood abuse may be at particularly high risk for HIV. Childhood abuse, and in particular associated cognitive, emotional, and social impairments, in people with SMI is directly and indirectly related to HIV risk behavior with substance abuse and adult victimization as mediators (Meade et al. 2009).

A longitudinal analysis (Prince et al. 2012), exploring the relationships between diagnosis of SMI and subsequent new diagnoses of HIV among Medicaid beneficiaries in eight US states (N = 6,417,676), underscored the link between substance abuse and the risk of new HIV diagnoses in SMI patients. Among people with major depressive disorder, bipolar disorder, and schizophrenia, those with substance abuse or dependence were, respectively, 3 (adjusted OR = 3.04), 2.5 (adjusted OR = 2.45), and 1.6 (adjusted OR = 1.63) times as likely (p < 0.001) as those without substance abuse or dependence to be diagnosed with HIV during the next 3 years. These results therefore suggest once again that assessing and addressing substance abuse, as well as associated high-risk behaviors, are essential factors to reduce HIV/AIDS risk among persons with SMI. In contrast to what might be expected on the basis of earlier reports of associations between SMI and HIV risk, the authors did not find SMI diagnosis in the absence of substance abuse to be associated with increased risk of HIV/AIDS. People with SMI but without an SUD in 2001 were 23 % less likely (adjusted OR = 0.77, p < 0.001) than people without SMI or an SUD to receive a new HIV diagnosis during the next 3 years. Only major depressive disorder seemed to confer such risk (12 % increase, adjusted OR = 1.12, p < 0.01). After adjustment for substance abuse or dependence diagnosis and demographic and selecting other characteristics, the presence of bipolar disorder was not associated with higher odds of new HIV/AIDS diagnoses, and the presence of schizophrenia was even associated with lower odds of new HIV/AIDS diagnoses (OR = 0.56, p < 0.001). Prince et al. (2012) therefore conclude that it remains unclear whether behavioral factors associated with SMI, other than those captured by a substance abuse or dependence diagnosis, also increase the risk of HIV/AIDS diagnoses.

Nevertheless, because of the high HIV prevalence rates, it is important that dual disorder patients are tested for HIV. However, studies investigating HIV testing rates among individuals with an SMI indicate that fewer than half of these patients (percentages ranging from 17 % to 47 %) have been tested in the past year (De Hert et al. 2011a). Since many patients with SMI are exposed to atypical antipsychotics, which have been associated with metabolic abnormalities, and since patients infected with HIV and on highly active antiretroviral therapy may also develop metabolic abnormalities, this group of patients is at particularly high risk for developing the metabolic syndrome and ultimately cardiovascular diseases (Vergara-Rodriguez et al. 2009).

Across different continents, markedly elevated rates of hepatitis virus infection have been reported in persons with SMI, compared to the general population. Overall, an estimated 20–25 % of persons with SMI are infected with HCV (De Hert et al. 2011a). Several studies have shown that SMI patients with SUDs even have higher rates of HCV infection (Mistler et al. 2006; Huckans et al. 2006; Matthews et al. 2008). Matthews et al. (2008) collected retrospectively data on 325,410 patients from electronic medical records and compared HCV prevalence rates in bipolar disorder patients with and without SUDs (N = 9,750). Compared with a control group with no history of either bipolar disorder or SUD, patients in the dual disorder group (N = 4,724) had a 5.46-fold increase in the relative risk of HCV infection, followed by the SUD group without a bipolar disorder (N = 37,970) (4.86-fold risk increase) and the bipolar disorder group without an SUD (N = 5,026) (1.31-fold risk increase). Huckans et al. (2006), utilizing a Veterans Healthcare Administration medical record database, found that, of those tested for HCV, 31.1 % (943/3,029) of veterans with comorbid schizophrenia and SUD were confirmed to have HCV, compared with 9.9 % (219/2,207) of veterans with schizophrenia but no documented history of SUD. Respectively, these groups were approximately eight (OR = 8.12, 95 % CI:7.47–8.82, p < 0.001) and two times (OR = 1.98, 95 % CI: 1.71–2.28, p < 0.001) as likely as the control group of patients without these diagnoses to have HCV infection. As even patients in the schizophrenia group with no SUD history were twice as likely as those in the control group to have HCV infection, these results equally show that a diagnosis of schizophrenia may be a risk factor independent of SUD.

HCV infection is a major cause of liver disease, including cirrhosis and hepatocellular carcinoma (Loftis et al. 2006). The most common routes of HCV transmission for persons with SMI are drug-use behaviors and sexual behaviors related to drug use (Mistler et al. 2006). For example, increased risk of bipolar disorder patients for both HCV and its related hepatic morbidity may come from some patients’ participation in high-risk behaviors like intermittent/episodic drug use or hypersexuality when manic. In addition, AUDs are relatively common in bipolar patients, which may increase the likelihood of high-risk behaviors as well as increase risk of progression of liver disease secondary to alcohol use in those patients with HCV (Matthews et al. 2008). Rosenberg et al. (2001) found, in a large sample (N = 931) of patients with an SMI undergoing inpatient or outpatient treatment, that being positive for HCV was associated with several substance using variables, including the presence of an SUD (alcohol, cannabis, and cocaine), a lifetime history of injection drug use, a lifetime history of sniffing or snorting drugs, and a lifetime use of crack. Injection drug use, compared with those without injection drug use, increased the risk of HCV infection to more than 31-fold (OR = 31.25, 95 % CI: 18.47–49.52, p < 0.001). A study of Klinkenberg et al. (2003), trying to estimate the prevalence of HCV among homeless persons with dual disorders, found 29.8 % (34/114) were antibody positive for HCV. Substance use variables having a significant bivariate relationship with HCV status were having a history of injection drug use (p < 0.01) and needle sharing (p < 0.01). SMI persons with a history of injection drug use were about three times more likely (OR = 3.19) to have a reactive test for HCV as SMI persons without a history of injection drug use.

These results underline the centrality of SUD, particularly injection drug abuse, in HCV infection. Therefore, especially patients with dual disorders should have routine screening and treatment for HCV infection to prevent associated morbidity and mortality (De Hert et al. 2011a). Unfortunately, although there is an overwhelming body of evidence that HCV-infected patients with psychiatric and addiction comorbidities can safely and effectively undergo antiviral treatment with similar sustained viral responses, many dual disorder patients are left untreated. If these patients undergo therapy, it is important that such treatment is delivered within the context of a multidisciplinary setting. In particular multidisciplinary approaches that combine HCV treating providers with mental health, addictions, and other support systems can facilitate preparation and successful treatment of these patients (Bonner et al. 2012).