Table 2 Overview of included papers (n = 62^a)

Harm reduction (including needle/syringe exchange; safe/supervised injection consumption sites; naloxone training/distribution)

13

Ashford, Curtis and Brown (2018); Hayashi et al. (2010); Dechman (2015); Bardwell, Boyd, Kerr & McNeil (2018); Jozaghi & Reid (2014); Kennedy et al. (2019); Taylor H et al. (2019); Bardwell, Collins et al. (2017); Wright et al. (2006); Mitchell et al. (2017); Bardwell, Flemming, Collins et al. (2019); Poland et al. (2002); Collins et al. (2019)

Homelessness

15

Barker and Maguire (2017); Barker, Maguire, Bishop & Stopa (2018a); Barker, Maguire, Bishop & Stopa (2018b); Groundswell report Finlayson et al. (2016); Stewart et al. (2009); Parkes et al. (2019); Hunter & Power (2002); Pakhale, Kaur, Charron et al. (2018); Goldade et al. (2012); Charron et al. (2018); Pakhale, Kaur, Florence et al. (2016); Bardwell, Collins et al. (2017); Bean, Shafer & Glennon (2013); Crisanti et al. (2017); Tsai and Rosenheck (2012)

Abstinence-based programmes including AA/12 Step; relapse; and recovery

12

Blondell et al. (2001); Rayburn & Wright (2010); Boisvert et al. (2008); Tracy et al. (2012); Tracy, Guzman & Burton (2014); Tracy and Wallace (2016); Rosenblum et al. (2005); Ashford, Curtis and Brown (2018); Chapman et al. (2018); Ashford et al. (2019); Eddie et al. (2019); Davidson et al. (2010)

Smoking cessation

4

Pakhale, Kaur, Charron et al. (2018); Goldade et al. (2012); Charron et al. (2018); Pakhale, Kaur, Florence et al. (2016)

Physical health including: Tuberculosis, Hepatitis; and HIV

12

Croft, Hayward & Story (2013); Hirsch-Moverman et al. (2013); Deering et al. (2009); Nyamathi et al. (2001); Weeks et al. (2006); Swendeman et al. (2019); Latkin et al. (2003); Taylor J et al. (2019); Nyamathi et al. (2015); Tookey et al. (2018); Stagg et al. (2019); MacLellan et al. (2017)

Sub-populations including: veterans; people who have been in prison and criminal justice experienced individuals

11

Tsai and Rosenheck (2012); Ellison et al. (2016); McCarthy et al. (2018); Simmons et al. (2017); Resnick and Rosenheck (2008); Hebert et al. (2008); Krawczyk et al. (2019); Nyamathi et al. (2015); Lennox et al. (2017); Gonzalez et al. (2019); Bellamy et al. (2019)

Other

2 (history of development of peer support; 3 separate studies commentary)

Power (2002); Gardien and Laval (2019)