In the end of 2019, the United Nations High Commissioner for Refugees (UNHCR) estimated a number of 79.5 million forcibly displaced people, i.e., people who are driven from their homes by persecution, conflict, violence, human rights violations, or events seriously disturbing public order. This is a record high, which accelerated in recent years as a response to the armed conflicts in Syria, but also conflicts in other areas like the Middle East, parts of sub-Saharan Africa, and the persecution of the Rohingya people in Myanmar (1). In 2019, the largest single group of people asking for asylum in Germany were of Syrian origin (27.3%, n = 39.270), followed by people of Iraqi origin (9.5%, n = 13.742) ranked second, Turkish origin (7.4%, n = 10.784) ranked third, Afghan origin (6.7%, n = 9.522) ranked fourth, Nigerian origin (6.2%, n = 9.070) ranked fifth, and Iranian origin (5.7%, n = 8.407) ranked sixth (2). On the basis of the 1951 Refugee Convention, a refugee is protected by international law and defined as “someone who is unable or unwilling to return to their country of origin owing to a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group, or political opinion” [(3), p. 3]. On their website, the UNHCR further describes refugees as “people who have fled war, violence, persecution and have crossed an international border to find safety in another country” [(4), para. 1]. For ease of reading, we will use the word refugee interchangeably with the terms forcibly displaced people and forced migrants.

There is an urgent need to support the mental health of refugees (5), who are in a particularly vulnerable situation due to exposure to trauma and other adverse health effects before migrating from their homes in conflict-affected settings, during their journey, and upon arrival in intermediate and final receiving countries (6, 7). Though estimates regarding refugees' mental health vary widely, a growing number of studies and meta-analyses suggest an elevated risk for mental health disorders in refugees [for an overview see Henkelmann et al. (8), Silove et al. (9), and Crepet et al. (10)]. New WHO estimates adjust previous estimates upwards, showing a high prevalence of mental health disorders among people affected by conflict and war. Of these, one person in five (22.1%) lives with some form of depression, anxiety disorder, post-traumatic stress disorder (PTSD), bipolar disorder, or schizophrenia, compared to the general population in which “only” one person in 14 with any mental disorder worldwide does (11). The Federal Chamber of Psychotherapists in Germany (12) estimated in 2015 that 40–50% of refugees in Germany suffer from PTSD or depression. Of the (mostly West African) refugees who arrived in Italy in 2015, ~89% experienced possibly traumatic events during their often partly seaborne migration (10). Less data on experienced trauma is available for refugees arriving to Greece crossing the Mediterranean Sea, the majority of refugees having fled Syria (57%), Afghanistan (22%), and Iraq (5%) (until mid 2015) and most of which wanting to settle in Germany or Sweden (13).

In the general population, evidence from longitudinal (14–16) and experimental studies (17, 18) suggests that certain mental health disorders, such as PTSD, previous alcohol or drug use, depressive or anxiety disorders, predict later alcohol abuse or dependence and illicit drug abuse and dependence. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), merges substance abuse and substance dependence into a single classification of substance use disorder (SUD). On a continuum from mild to severe, it describes “a cluster of cognitive, behavioral, and physiological symptoms indicating that the individual continues using the substance despite significant substance-related problems” [(19), p. 483]. Problematic, harmful, or hazardous substance use describes an absence of a current substance-related disorder, however, increases the risk of harmful consequences, including physical, mental, and social problems for the user or others (20). It predicts the development of substance use related harm such as SUD, social problems, and the utilization of health care services (21).

According to the United Nations Office on Drug and Crime World Drug Report (22), risk factors for substance use are a refugee status, poverty, conflict/war, homelessness, social exclusion and inequality, neighborhood disorders, peer substance use and drug availability, mental health problems, trauma and childhood adversity. Problematic use of substances and the transition to SUD are known to have a higher prevalence in people with a lower socioeconomic status [SES; e.g., Legleye et al. (23)] as well as in people with certain mental disorders (14–18).

Initial research suggests that displacement experiences, their potential consequences and conditions in hosting countries (e.g., uncertain future prospects) may increase vulnerability to substance use, its impact and the transition to more harmful patterns of substance use (24). From an epidemiological viewpoint, substance use and its harmful consequences are common among refugee populations. For example, systematic reviews identify prevalences of problematic alcohol use between 17 and 36% in camp settings and 4–7% in community settings (25) and high levels of opioid use due to forced displacement that is caused by armed conflict in Pakistani and Afghan populations (26). Initial research on the drug use and special needs of refugees in Germany shows that 28.4% of a sample of predominantly Iranian and Afghan refugees interviewed in low-threshold addiction services have started using alcohol and/or illicit drugs after leaving their home country. 16.4% of the interviewed people were occasionally consuming alcohol and cannabis, the most-used substance being opioids (27). According to qualitative research based on expert interviews, substance-using unaccompanied minor refugees are primarily using alcohol or cannabis and have started using after arrival in Germany (28). Although remaining under the national average, data from the transcultural psychiatric department in Vienna, Austria, shows an elevated alcohol consumption from patients with muslim background from the middle east, compared to the average consumption in their countries of origin (29). The opposite was shown for cannabis, where the reported consumption remained under the expected value, based on the comsumption pattern in the countries of origin. German law prohibits cannabis acquisition and possession while alcohol remains legal for adults. Increased alcohol consumption and decreased cannabis consumption among refugees might be heavily influenced by legal regulations and social norms in the country of stay. Generally, it was found that regardless of former protective factors such as religious background, consumption patterns adapt to the social majority ones when length of stay increases [e.g., Arfken et al. (30); Harris et al. (31)]. It still remains questionable to what extend protective effects of cultural and social norms can be attributed to the subgroup of forced migrants (25). Next to research on protective factors, further research is required to assess key risk factors, patterns of use, and the extent of conflict-associated harmful use of e.g., alcohol beyond a merely suggestive causal relationship (32, 33).

These results, including the high number of Iranian and Afghan refugees who have started using alcohol and illegal substances after leaving their home country or upon arrival in Germany and, furthermore, the elevated risk for a transition to more harmful patterns of substance use, highlight the need for low-threshold early and indicated prevention targeting (young) refugees; Especially since asylum seekers in Germany tend to become younger over the last 5 years, with a growing majority being underage and a second largest group being between 18 and 25 years old (34). Indicated prevention describes interventions in individuals who show a manifest risky or problematic behavior (35), such as problematic/hazardous substance use in the absence of SUD.

Despite a high risk for the development of mental health disorders, refugees have legal access to just very restricted areas of general health care in Germany (36) and, regarding mental health care in receiving countries, face numerous structural and socio-cultural barriers that limit accessibility and utilization of services (37).

Digital health interventions (DHI) can address some of these barriers and can thus have an impact on the delivery of addiction prevention or harm reduction to marginalized populations. Structurally, DHI have the potential to reduce barriers such as a lack of resources, language barriers, and bureaucracy. As an example, Calvo et al. (38) demonstrated the accessibility of a smartphone app aimed at reducing harm associated with intravenous drug consumption among homeless drug users. Socio-culturally, DHI work with a high level of privacy and can reduce fear of stigma and discrimination (39), which would otherwise represent major barriers for individuals with substance use problems to seeking treatment (40). Furthermore, they may lower the impact of shame on utilization of services and bypass the necessity to ask for help personally; all of which were commonly seen as barriers to accessing traditional mental health care in Syrian refugees (37).

The majority of Syrian and Iraqi refugees in Germany have had access to a smartphone during migration (78 and 87%, respectively), do so upon arrival in Germany (89 and 92%, respectively), and use the internet daily (86 and 82%, respectively) (41). Therefore, we assume a high accessibility of smartphone-based DHI, e.g., apps, for the present study's target group. This, however, under two assumptions: first, that Afghan, Iranian, Iraqi, and Syrian refugees in Germany show a similar behavior toward apps and their usage in terms of needs and concerns as the German population. Second, that after cultural adaptations, content meets these needs also in terms of values, worldview, and cultural customs of the targeted population (42, 43). Otherwise, new barriers to accessing digital health care may arise (e.g., lack of technical literacy, difficulties accessing the internet, a doubt in the credibility, or low acceptance of services) (44). In terms of effectiveness, DHI have shown to be effective in reducing problematic use of alcohol compared to either no intervention or merely providing general health information and show similar effects as face-to-face interventions (45). The clinical significance for stand-alone and thus purely self-guided DHI is less clear compared to blended and therapist-guided digital approaches (46). As a strength—however—they may present a viable approach to the lack of language skills and cultural competencies of clinicians, but most importantly to lacking legal access to mental health care and unclear cost coverage for refugees in intermediate or final receiving countries, e.g., Germany. This leaves evidence-based stand-alone DHI a scalable and low-threshold service in reaching marginalized populations, such as refugees, who would face difficulties to make use of mental health services otherwise.