Home > Evidence review of drug treatment services for people who are homeless and use drugs.

Galvin, Brian (2021) Evidence review of drug treatment services for people who are homeless and use drugs. Drugnet Ireland, Issue 77, Spring 2021, pp. 21-26.

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The Irish national drugs strategy aims to improve access to treatment services for people who are homeless who use drugs and have complex needs. On behalf of the Department of Health, the Health Research Board commissioned the Salvation Army Centre for Addiction Services and Research (SACASR), the University of Stirling, and the Public Health Institute at Liverpool John Moores University to systematically review and synthesise the international evidence on the efficacy of interventions designed to serve this population. This synthesis will inform policies, currently under review, regarding the provision of services to people who are homeless.

This report1 comprises two parts: the first part presents a description of the current trends relating to drug use and of the services in Ireland in primary care, mental health, and drug treatment settings for people who experience homelessness and use drugs; the second part is an integrative review of the international research literature providing a systematic evaluation of the evidence on interventions that aim to address the needs of this population.


People who are homeless have complex and challenging lives. They tend to have worse physical and mental health and are more likely to report problem substance use than the general population. Substance use is more prevalent among people who are homeless than in the general population and providing support services and drug treatment in a holistic way for this population should be a priority. Increasing the provision of evidence-based support may lead to improvements in health, wellbeing, and quality of life (QoL), and to a reduction in costs to healthcare and wider public services.

Overview of services in Ireland for people who are homeless and use drugs

There are a wide range of services in Ireland for people who are homeless and use drugs. These include health and social care services specific to substance use, such as counselling; drop-in centres; assessment and intervention advice; information and education; ongoing support; follow-up care; and a drug-screening facility. Outreach clinics are also available across Ireland. Specific programmes relevant for people who are homeless include the Health Service Executive (HSE) Social Inclusion programme, which is designed specifically for marginalised groups to help enable and improve their access to mainstream services. For example, the Inclusion Health Service at St James’s Hospital, Dublin attends to the complex needs of marginalised groups, such as people who are homeless and those who use drugs. Other existing services include abstinence-based drug treatment services, such as residential rehabilitation and harm reduction drug services (e.g. prescribing services, including opioid substitution therapy – OST); and static, pharmacy, and outreach needle and syringe programmes services. There are also housing support services across Ireland, such as transitional housing and emergency accommodation provision. It is important to note that many nationwide services provide a holistic approach to treatment and support; therefore, it is not possible to fit them neatly into service type. In particular, many third-sector organisations offer a range of services in order to best meet people’s needs. These can include drug treatment services, other harm reduction services, housing support services, and other more general health and social care services, among others.

Evidence review on effective interventions for people who are homeless and use drugs

This report is a systematic review of reviews and aimed to synthesise international evidence on effective interventions for this population. Twenty-two publications (18 published papers and four grey literature reports) published between 2004 and 2020 were included. Thirteen of the 18 publications included academic reviews deemed to be systematic (with two of these also including a meta-analysis) and five were deemed to be non-systematic. Twelve of the reviews included quantitative studies only; eight included different study type/mixed designs (including one realist synthesis); one presented a meta-ethnography and included qualitative studies only; and one was a review of reviews. Ten of the reviews were undertaken in the United Kingdom (UK); four in the United States of America (USA); four in Canada; three in Europe (Spain, Ireland, and a Dutch/Belgian collaboration); and one was an international collaboration by researchers from Switzerland, the UK, and Canada. Despite this, nearly all of the reviews (n=19) were international in focus, although two reviews focused on the USA only and one focused on the UK only. Even though the focus of most of the reviews was international, the majority of the authors were based in the UK, and the majority of primary studies were undertaken in the USA. This may affect the generalisability of the findings to non-USA contexts.

The focus of the included reviews varied, and a large number of interventions were investigated. The largest number of reviews (n=6) focused on housing interventions, including Housing First (HF) initiatives. Of the remaining reviews, four focused on interventions for people with co-occurring serious mental health and alcohol/drug issues (COSMHAD); three focused on substance use treatment specifically; two investigated healthcare treatments and interventions in general; two focused specifically on case management interventions; one focused solely on assertive community treatment (ACT); one focused on sexual health promotion interventions; one investigated the impact of harm reduction interventions on the incidence of hepatitis C virus; one examined the effectiveness of intentional peer support (IPS) for people who are homeless; and one examined emergency-department-based interventions. The primary outcomes of interest were treatment engagement and retention as well as successful treatment completion. The study also synthesised information relating to substance use outcomes, housing outcomes, and ‘other’ outcomes (primarily health and wellbeing outcomes).

Treatment engagement and retention

Treatment engagement and retention for the homeless population can be problematic regardless of intervention type. ACT can lead to increased engagement rates for people who experience homelessness and use drugs. In contrast, treatment engagement with intensive case management (ICM) can be low, with more than two-thirds of participants experiencing both substance use problems and homelessness who enrol in shelter-based ICM services dropping out of these programmes. There is some evidence to suggest that motivational interviewing and motivational enhancement therapy can increase treatment engagement during the short term for those experiencing homelessness and COSMHAD. Adherence to highly active antiretroviral therapy (HAART) among people who use drugs is comparable to that among people who do not use drugs; however, the addition of OST to HAART for those who use drugs increases treatment adherence and leads to better treatment outcomes. Data from studies of HF interventions suggest that engagement can be difficult, possibly due to the fact that while supported and encouraged through a harm reduction approach, treatment engagement within HF is ultimately self-determined.

Finally, there is evidence suggesting that the way in which interventions are delivered can play a crucial role in treatment engagement and retention, with compassion, warmth, and a lack of judgement and stigma from the staff supporting individuals being paramount.

Successful treatment completion

There is a lack of studies reporting on successful treatment completion, while (limited) data were only presented in two of the included reviews. One low-quality review presented evidence from four randomised controlled trials and one meta-analysis of a linear, rigorous abstinence-contingent housing approach (called ‘Birmingham house’), which suggests that treatment completion rates in such an approach (65% in the most recent trial) can be higher than the approximate 50% for social interventions (such as case management, congregate living, and vocational training), and comparable to those of modified therapeutic communities. There is some evidence that integrated approaches in short-term residential programmes (lasting six months or less) for people with COSMHAD were associated with higher rates of programme completion. Moreover, there was evidence that monetary and non-monetary incentives can increase completion rates of directly observed preventive therapy in young people with latent tuberculosis who are homeless; however, it was not specified whether they were also experiencing problem substance use. Also, there was some evidence that for people experiencing homelessness who also inject drugs, an accelerated hepatitis B virus immunisation schedule (with doses administered at 0, 7, and 21 days, and a booster at 12 months) can result in superior completion rates, compared with traditional schedules that have similar seroconversion rates.

Evidence shows that monetary and non-monetary incentives can increase completion rates of directly observed preventive therapy in young people with latent tuberculosis who are homeless; however, it was not specified whether they were also experiencing problem substance use. Lastly, integrated approaches in short-term residential programmes (lasting six months or less) for people with COSMHAD were associated with higher rates of programme completion.

Treatment outcomes: substance use

All the included reviews reported on some element of substance use outcomes and, overall, the results were mixed. First, there is a large number of intervention types available for people experiencing homelessness with concurrent substance use problems. In general, the greater the level of integration and partnership between programmes and agencies dealing with people who are homeless and have co-occurring substance use problems, the better the outcomes.

Evidence suggests that harm reduction interventions can lead to decreases in drug-related risk behaviour (e.g. needle sharing) in this population in the same way as they do for other groups. Co-locating a number of harm reduction approaches together (termed ‘full harm reduction’) creates additional opportunities for clients that can lead to better outcomes than single (partial) harm reduction interventions. The reviews suggested that HF does not seem to impact either positively or negatively on substance use outcomes.

Treatment outcomes: housing

Reviews which reported on housing outcomes largely support the HF approach in terms of its effectiveness in increasing housing stability and retention, and indicate the HF approach as a preferred option due to the flexibility and harm reduction ethos associated with it. However, the reviews identified some issues relating to programme fidelity and type of HF housing (scattered versus single site). There is also some evidence that supportive housing can have a positive effect on housing stability. Other non-housing-specific interventions can also have a positive effect on housing outcomes. Most notably, peer support interventions, with IPS specifically being assessed, can lead to a decrease in the number of homeless days and a reduction in relapse to homelessness. Evidence regarding case management interventions and their impact on housing outcomes is mixed and varies between intervention types.

Treatment outcomes: other

Some treatment outcomes that were not related to housing or substance use were reported. These related primarily to mental health and wellbeing outcomes, with mixed evidence regarding the effectiveness of the different interventions studied. There is some evidence from interventions delivered in the USA that permanent supportive housing for people experiencing homelessness and who have additional mental health problems can lead to a reduction in mental health symptoms, compared with a control condition. There is strong evidence that HF can improve measures of physical health in the short term for ‘housing-vulnerable’ adults. This included moderate-strength evidence for positive effects on personal wellbeing, mental health, and locality-related wellbeing (i.e. wellbeing related directly to one’s living situation and conditions), with no effects on personal finance or community wellbeing being reported. There is some evidence that the HF congregate model (where all residents live in one apartment block) can lead to greater improvements in mental health and QoL than the scattered HF model (where residents live in various individual locations). Lastly, there is evidence that integration of services and holistic treatment for people with comorbidities and COSMHAD leads to better psychosocial and substance use outcomes.

Policy and research recommendations

People who experience homelessness and problem substance use are a population with complex needs. However, it is important to note that they are not a homogenous group. Individuals will be dealing with severe challenges by virtue of being homeless, but may also be facing concurrent issues relating to substance use. There are gaps in monitoring and other routinely reported data that would provide better insights into the needs of this population and the harms associated with substance use. This includes regular prevalence surveys; data on drug-related deaths in people who are homeless; and infectious disease and blood-borne virus prevalence monitoring in people who inject drugs, including data on housing needs.

A substantial evidence base exists regarding effective interventions for people who experience homelessness and for people with problem substance use, but not enough research has been conducted focusing on the unique needs of people who experience both, despite these issues commonly co-occurring. To ensure needs are well met, targeted provision can be helpful and specific subgroups do exist within this wider population, for example, people who experience homelessness and COSMHAD, whose needs can be even more complex. Other identity characteristics, such as gender, age, ethnic background, and experiences of physical disability/physical health problems, will also have an impact on a person’s needs, preferences, and overall treatment experience. Unfortunately, there are few studies that focus on making sure that people with these very complex and challenging experiences are well heard.

While people who experience homelessness and problem substance use experience different circumstances and have different needs, wants, and preferences, which are also likely to change over time, making listening to individuals and providing choice is critically important. A balance is therefore needed between providing an approach that is tailored specifically to each individual and delivering key components of evidence-based services and interventions. Currently, there is a lack of standardisation of measures and outcomes, which can make meaningful comparisons between different types of service models – and, subsequently, any distillation of key elements of success – challenging.

Regarding specific intervention types, the evidence suggests that the HF model supports a flexible harm reduction approach that enables referral to other services needed by the residents. The evidence base strongly suggests consistent positive housing outcomes and the absence of negative effects on substance use, alongside some evidence for positive effects regarding physical health and wellbeing. The review found that case management-type interventions can be effective, both when applied on their own and when combined with other interventions, such as contingency management, positive reinforcement or incentives, art therapy, and health prevention and promotion programmes. ACT has consistently produced positive effects on housing stability and has been found to be cost-effective, but this model seems to be suitable mainly for those experiencing homelessness and COSMHAD. Finally, the evidence suggests that formal IPS can lead to positive housing, substance use, and wellbeing outcomes, and that it has the potential to have a positive impact on the peers who provide the support.

For this reason, the authors of the study recommend the development of peer support interventions for people who experience both homelessness and problem substance use. However, due care must be given to planning for the embedding of peers in services in order to ensure that they are respected, valued, and offer meaningful support and training opportunities.

Key messages

  • A lack of international research exists on effective interventions for people who are homeless and use drugs. There is also a lack of Irish research, in particular which examines this intersection in depth. 
  • People who are homeless and use drugs are not a homogenous population. More research, particularly qualitative research, should be conducted to explore the ‘missing voices’. 
  • Treatment for this population group should be needs-led and person-centred. However, most research has examined complete treatment interventions and service models, with a lack of evidence on the effect of tailoring these. 
  • Treatment failure often stems from the service providers not recognising the breadth and complexity of individual needs. 
  • Mindful of choice for service users, a flexible system which provides opportunities for both harm reduction and abstinence-based approaches is recommended. 
  • Consensus on outcome measures (including treatment outcomes and treatment completion indicators for this population) should be reached to help research standardisation and support meaningful comparisons between interventions. 
  • Research is required on the optimal length of treatment for this population. This has implications for practice, as research findings may lead to a need to secure funding for extended periods of treatment. 
  • The findings highlight the importance of integration between different services, especially for people who are homeless and who experience COSMHAD.  
  • How interventions are delivered (e.g. non-judgemental, compassionate), providing choices, and respecting service users’ preferences for approach is an important determinant of success. 
  • In the reviewed research, there are insufficient data on treatment retention and completion available and/or synthesised. 
  • Housing interventions, especially HF, lead to improvements in housing outcomes, but evidence regarding HF interventions and health and wellbeing outcomes is mixed. Evidence suggests that HF does not impact on substance use outcomes. 
  • Case management-type interventions can be effective, but ACT seems suitable primarily for those experiencing homelessness and mental health problems or COSMHAD. 
  • Peer support interventions can lead to positive housing, substance use, and wellbeing outcomes, but care must be taken when embedding peers into services due to common challenges experienced in such roles.


1  Miller J, Carver H, Masterson W, Parkes T, Jones L, Maden M and Sumnall H (2021) Evidence review of drug treatment services for people who are homeless and use drugs. HRB Drug and Alcohol Evidence Review 7. Dublin: Health Research Board. https://www.drugsandalcohol.ie/33910/

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