Table 1.

Studies on access, engagement, and outcomes related to traditional OUD treatment for PEH.

Author(s) (Year) Location	Study Design	Population	Relevant Outcomes
Alford et al. (2007) Boston, MA	Cohort study	Homeless and housed individuals dependent on opioids	The number of patients who left treatment and the reasons for leaving treatment before 12 months, including the treatment failure and successful program departure groups, appeared similar for both homeless and housed patients. Over the 12-month period, no significant associations were found between housing status and use of opioids or other drugs.
Amodeo et al. (2004) Massachusetts, USA	Cohort study	Female individuals who inject drugs and reported heroin as primary drug	Homeless females who injected drugs were about 80 % more likely than non-homeless females who injected drugs to use detoxification only (Odds Ratio [OR] = 1.804, 95 % CI 1.538, 2.116).
Bachhuber et al. (2015) USA	Cohort study	Veterans initiating methadone or buprenorphine treatment	The prevalence of homelessness was 10.2 %, and 5.3 % were at risk for homelessness. Male (vs. female) veterans (10.3 % vs. 8.9 %) and veterans ages 18−34 and 45−54 (12.0 % and 11.7 %, respectively) more frequently screened positive for homelessness.
Bauer et al. (2016) Boston, MA	Cohort study	Homeless adults who died of drug overdose	Fewer than half of decedents with documented opioid use had received pharmacological treatment for opioid dependence. In particular, treatment with buprenorphine (4.3 %) and methadone (19.6 %) rarely occurred.
Chang (2017) San Francisco, CA	Qualitative study	Formerly homeless women living in supportive housing	Participants described being tethered to the Tenderloin because they needed to access substances or treatment, including substitution therapy. One participant was on methadone maintenance treatment for OUD and felt geographically tied to the neighborhood due to requirements that she obtain methadone treatment on-site at the clinic daily.
Corsi et al. (2007) Denver, CO	Cohort study	People who inject opiates	Not considering oneself to be homeless was significantly associated with treatment entry (i.e. methadone maintenance, outpatient drug-free, or residential treatment). Not being homeless independently predicted treatment entry (OR = 1.67, P < 0.05).
Cousins et al. (2016) Los Angeles, CA	Cohort study	Patients who obtained XR-NTX for opioid dependence	Significantly more patients in XR-NTX treatment who used heroin identified as homeless compared to those in XR-NTX treatment who used non-heroin opioids (P < 0.05). Identifying as homeless was associated with a lower odds of returning for 2 or more doses of XR-NTX; however, this association was not significant (OR = 0.74, 95 % CI 0.34, 1.61).
Damian et al. (2017) Baltimore, MD	Cohort study	Individuals dependent on opioids	Clients who reported unstable housing, including being homeless, residing in a treatment house, or in a transitional house, had a 41% decreased odds of remaining at least 90 days (i.e. higher odds of treatment failure) compared to clients who lived independently at intake (OR = 0.59, 95 % CI 0.37, 0.96).
Daniulaityte et al. (2019) Dayton, OH	Cross-sectional study	Adults with OUD who used non-prescribed buprenorphine	Past 6-month homelessness significantly differed between classes. The "Intense Non-Prescribed Buprenorphine Use" class had the lowest prevalence of homelessness (27.3 %), while the "Heavy Heroin/Fentanyl Use" class had the highest prevalence of homelessness (58.9 %). The "More Formal Treatment Use" class had a 55.4 % prevalence of homelessness.
Deck and Carlson (2004) Oregon and Washington, USA	Cohort study	Medicaid-eligible adults entering treatment for opiate use	Homeless individuals (vs. other) were significantly less likely to obtain access to MMT in both Oregon (OR = 0.29, P < 0.01) and Washington (OR = 0.55, P < 0.01).
Dunn et al. (2019) USA	Cohort study	Patients initiating detoxification and outpatient OUD treatment	Patients in detoxification had lower odds of receiving planned OAT if they were homeless, as compared to having a dependent/independent housing status (Adjusted Odds Ratio [AOR] = 0.53, P < 0.001).
Englander et al. (2020) Portland, OR	Cohort study	Inpatients receiving addiction consultation services	Current homelessness was significantly associated with initiation of MOUD or medications for alcohol use disorder (AOR = 2.63, 95 % CI 1.52, 4.53).
Eyrich-Garg et al. (2008) Urban Areas in USA	Cohort study	Individuals seeking treatment for substance misuse	Inpatient/residential treatment clients were more likely to be literally homeless and marginally housed while outpatient and methadone maintenance clients were more likely to be housed (P < 0.0001).
Fine et al. (2020) Boston, MA	Cohort study	Adults who engaged in buprenorphine treatment	Homelessness was independently associated with an increased hazard of all-cause mortality (Adjusted Hazard Ratio [AHR] = 1.39, 95 % CI 1.09, 1.78) and an increased hazard of opioid overdose-related mortality (AHR = 1.77, 95 % CI 1.25, 2.50).
Havens et al. (2009) Baltimore, MD	Randomized controlled trial	People who inject drugs requesting OAT referrals	Being homeless and not living in one's own apartment/home were not significantly associated with number of days in OAT.
Hoffman et al. (2019) Six Sites in USA	Qualitative study	Research staff and providers for an XR-NTX clinical trial	Homelessness impeded prospective participants' ability to engage in the study. Turbulent living conditions were considered to be a universal barrier to recruitment. One clinician explained that in her clinic, recruiters "might have hooked in with somebody but then the housing falls through and then we lose them."
Jones et al. (2017) USA	Cohort study	Patients admitted for prescription opioid misuse	Compared to living independently, being homeless was associated with increased relative risk of injection (Relative Risk Ratio [RRR] = 1.73, 95 % CI 1.69, 1.77) and smoking or inhalation (RRR = 1.14, 95 % CI 1.12, 1.17) as the usual route of opioid misuse compared to oral misuse.
Kelly et al. (2018) USA	Cohort study	Veterans with OUD at Veterans Health Administration facilities that prescribed XR-NTX	Veterans who received XR-NTX were more likely to have been homeless in the past year compared to those who received OAT (AOR = 1.52, 95 % CI 1.07, 2.16). Veterans with a prescription for XR-NTX were more likely to have been homeless in the past year compared to those who received no OUD medication (AOR = 1.58, 95 % CI 1.24, 2.19).
Kertesz et al. (2003) Boston, MA	Cohort study	Patients admitted to inpatient detoxification	Compared to homeless persons not using stabilization programs, homeless stabilization program users were less likely to report heroin as their substance of choice (11 % vs. 29 %, P = 0.05). Compared to those without a recent experience of literal homelessness, homelessness was associated with later first use post-detoxification among subjects with heroin as their substance of choice (HR = 0.47, 95 % CI 0.23, 0.95).
Krawczyk et al. (2020) Maryland, USA	Cohort study	Adults admitted to outpatient treatment programs for OUD	Homeless individuals were significantly less likely to ever have received methadone or buprenorphine treatment (AOR = 0.46, 95 % CI 0.43, 0.50).
Krull et al. (2011) USA	Cross-sectional study	Addiction treatment program directors	Program directors that had less positive attitudes toward buprenorphine as an effective treatment for opiate dependence when they worked in organizations that served a higher percentage of people who were homeless (P < 0.05).
Li et al. (2019) Boston, MA	Case-control study	Veterans admitted for inpatient opioid detoxification	36.8 % were homeless at the time of index admission. 26.5 % of the patients alive at time of follow-up were homeless at the time of index admission compared with 47.1% of the deceased patients. Being homeless at index was trending and associated with a higher all-cause mortality but not significantly (P = 0.09).
Lundgren et al. (2003) Massachusetts, USA	Cohort study	People who inject drugs	Homelessness was positively associated with solely using detoxification (OR = 1.36, 95 % CI 1.27, 1.46) and with using residential treatment (OR = 2.75, 95 % CI 2.55, 2.97) but negatively associated with enrolling in MMT (OR = 0.49, 95 % CI 0.46, 0.53).
Marienfeld and Rosenheck (2015) USA	Cohort study	Veterans using MMT and/or living with serious mental illness	Patients enrolled in MMT who had at least 1 serious mental illness were more likely to have experienced homelessness than just those in MMT (RR = 1.52, P < 0.001) and much more likely to have experienced homelessness than just those with at least 1 serious mental illness (RR = 3.95, P < 0.001).
Masson et al. (2002) San Francisco, CA	Cohort study	Patients who used medical services and had a diagnosis associated with complications of opioid use	Compared to those who were not homeless, patients experiencing homelessness had greater mean emergency department visits (P < 0.001) and inpatient service (P < 0.0001) use. Patients experiencing homelessness also had higher mean service charges associated with their emergency department (P < 0.02) and inpatient service (P < 0.01) use.
Midboe et al. (2019) USA	Cohort study	Veterans who accessed homeless programs in the Veterans Health Administration	38 % of veterans with OUD received MOUD from the Veterans Health Administration within a year following program entry. Receiving MOUD after program entry was significantly more likely for veterans ages 35 and younger (P < 0.001) and for those without high-dose opioid (P < 0.001) or concomitant opioid-benzodiazepine prescriptions (P = 0.016).
Nyamathi et al. (2004) Los Angeles, CA	Cohort study	Women experiencing homelessness	Recent substance use treatment was not significantly associated with a motivation to quit for people who used heroin.
Patel et al. (2020) Nashville, TN	Descriptive report	Discharged patients with SUDs	Of the recovery houses that accepted discharged patients from Vanderbilt Medical Center in 2018, 12.5 % allowed patients to remain on buprenorphine/naloxone, 0 % allowed patients to remain on MMT, and 19 % completely allowed naltrexone maintenance therapy.
Reynoso-Vallejo et al. (2008) Massachusetts, USA	Cohort study	Latino men who inject drugs	Homelessness was associated with a decreased likelihood of entering methadone maintenance (OR = 0.41, 95 % CI 0.36, 0.47).
Rivers et al. (2006) USA	Cohort study	Individuals admitted for substance use treatment	Among both users of heroin and non-users respectively, those who reported dependent living (AOR = 1.90, 95 % CI 1.70, 2.13; OR = 2.23, 95 % CI 1.50, 3.31) and independent living (AOR = 2.11, 95 % CI 1.90, 2.34; OR = 2.73, 95 % CI 1.88, 3.98) were more likely to be planned for MMT than those who were homeless.
Riggins et al. (2017) Ten Sites in USA	Cohort study	HIV-infected patients receiving buprenorphine treatment	Homelessness was significantly associated with a decreased odds of self-reported opioid use at any follow-up visit (OR = 0.57, 95 % CI 0.34,0.96).
Robbins et al. (2010) San Francisco, CA	Cohort study	People experiencing homelessness who inject drugs	Needing health care 6+ times (OR = 2.12, 95 % CI 1.23, 3.68) and seeking health care (OR = 3.39, 95 % CI 1.77, 6.52) were associated with higher odds of methadone treatment. Methadone treatment was independently associated with increased odds of seeking care (AOR = 2.29; 95 % CI 1.24, 4.24).
Rose-Jacobs et al. (2019) Boston, MA	Cohort study	Pregnant women being treated for OUD	61 % reported housing instability. Compared to those with neither food nor housing instability, those reporting both food and housing instability had greater depressive scores (P = 0.02) and clinically but not statistically significant higher intimate partner vulnerability scores.
Royse et al. (2000) Five Sites in USA	Cohort study	Out-of-treatment people who use substances	Non-homeless individuals were more likely to have participated in methadone detoxification (OR = 1.55, P = 0.006) and methadone maintenance (OR = 1.86, P = 0.000) than those who reported homelessness.
Shah et al. (2000) Baltimore, MD	Cohort study	People who inject drugs	Regardless of HIV status, not being homeless in the past 6 months was associated with enrollment in MMT (P < 0.05). Homelessness in the past six months was only negatively independently correlated with decreased odds of MMT among HIV-negative subjects (OR = 0.72, 95 % CI 0.55, 0.95). Among those who seroconverted during follow-up, those who reported prior 6-month homelessness were less likely to subsequently enroll in MMT (AOR = 0.05, 95 % CI 0.01, 0.28).
Simon et al. (2017) Seattle, WA	Cohort study	Patients seeking treatment at an office-based opioid treatment program	Recent homelessness was associated with a decreased odds of reaching induction (AOR = 0.32, 95 % CI 0.10, 1.02).
Stein et al. (2017) Fall River, MA	Cohort study	Patients seeking inpatient opioid detoxification	Patients who preferred residential treatment after detoxification were more likely to be homeless (AOR = 5.71, 95 % CI 1.13, 28.95) than those who preferred no treatment.
Stein et al. (2015) Fall River, MA	Cohort study	Patients seeking inpatient opioid detoxification	Patients who preferred residential treatment after detoxification were more likely to be homeless than those who preferred OAT or those who preferred outpatient treatment. There was no evidence that the relationship between homelessness and treatment preference varied by season.
Stein et al. (2014) Fall River, MA	Cohort study	Patients seeking inpatient opioid detoxification	Homelessness was not significantly correlated with being uninsured.
Timko et al. (2016) USA	Cohort study	Veterans Health Administration patients with alcohol or opiate dependence	Homelessness was associated with receiving detoxification services (AOR = 3.09, 95 % CI 3.00, 3.19), a detoxification follow-up appointment (AOR = 1.62, 95 % CI 1.51, 1.73), and addiction treatment within 30 days (AOR = 1.69, 95 % CI 1.57, 1.81) and within 60 days (AOR = 1.93, 95 % CI 1.80, 2.07) of detoxification.
Upshur et al. (2018) Eleven Sites in USA	Cohort study	Homeless women with SUDs	Using heroin was associated with more reported use of drug-related services (OR = 1.89, 95 % CI 1.04, 3.47) and with attending Narcotics Anonymous/Cannabis Anonymous (OR = 1.91, 95 % CI 2.45, 67.59).
Van Ness et al. (2004) Central Harlem, NYC	Cohort study	People using crack or powder cocaine and/or heroin	Homelessness was not significantly associated with odds of using a methadone clinic.
Velasquez et al. (2019) New York City, NY	Qualitative study	Adults with OUD recently released from jail	Recent or chronic homelessness was described as a primary barrier to adhering to prescribed treatment (i.e. XR-NTX, methadone, and/or buprenorphine).