Table 1.

Quality assessment screening using the Mixed Methods Appraisal Tool.

Randomised Controlled Trials
First Author, Year	Are the Research Questions Clear?	Do the Collected Data Address the Research Questions?	Is Randomisation Appropriately Performed?	Are the Groups Comparable at Baseline?	Are There Complete Outcome Data?	Are Outcome Assessors Blinded to the Intervention Provided?	Did the Participants Adhere to the Assigned Intervention?	Comments
Eiroa-Orosa, 2010 [23]	Yes	Yes	Cannot tell	Yes	Can’t tell	Can’t tell	Yes	Does not describe how the randomisation schedule was generated.
Quantitative non-randomised
First author, Year	Are the Research Questions Clear?	Do the Collected Data Allow the Research Questions?	Are the Participants Representative of the Target Population?	Are Measurements Appropriate Regarding Both the Outcome and Intervention (or Exposure)?	Are There Complete Outcome Data?	Are the Confounders Accounted for in the Design and Analysis?	During the Study Period, Was the Intervention Administered (or Exposure Occurred) as Intended?	Comments
Abrahamsson, 2017 [18]	Yes	Yes	Yes	Yes	Yes	Cannot tell—see comment	Yes	Individuals were excluded if they had a prescription for pain, but 22% of the sample were prescribed pregabalin.
Durand, 2021 [21]	Yes	Yes	Yes	Yes	Yes	Cannot tell—see comment	Yes	Possible selection bias, as they excluded all in continuous MMT prior to study start date.
Eibl, 2019 [22]	Yes	Yes	Yes	Yes	Yes	Cannot tell	Yes
MacLeod, 2019 [24]	Yes	Yes	Yes	Yes	Yes	Cannot tell	Yes
Maremmani, 2014 [25]	Yes	Yes	Yes	Yes	Yes—per protocol	Cannot tell	Yes	Clinical setting, observational study, unable to follow up patients who dropped out.
Park, 2020 [28]	Yes	Yes	Yes	Yes	Yes	No	Yes	-
Rapeli, 2009 [29]	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Did not compare OAT vs. OAT and BZD.
Schuman-Olivier, 2013 [30]	Yes	Yes	Yes	Yes	Yes	No	Yes
Sharma, 2020 [31]	Yes	Yes	Cannot tell	Yes	Yes	Yes	Yes	A broader population, not just OAT patients.
Weizman, 2003 [32]	Yes	Yes	No	Yes	No	Yes	Yes
Xu, 2021 [33]	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Case–controlled.
Quantitative descriptive
First author, Year	Are the Research Questions Clear?	Do the Collected Data Allow the Research Questions?	Is the Sampling Strategy Relevant to Address the Research Question?	Is the Sample Representative of the Target Population?	Are the Measurements Appropriate?	Is the Risk of Nonresponse Bias Low?	Is the Statistical Analysis Appropriate to Answer the Research Question?	Comments
Bakker and Streel, 2017 [19]	Yes	Yes	Yes	Cannot tell (as it is a practice population)	Yes	Yes	Yes	Studied all OAT patients in one practice, not representative of wider population. Inferential tests limited to retention analysis (t-tests).
Best, 2002 [20]	Cannot tell	Cannot tell	Cannot tell	Cannot tell	Yes	No	Cannot tell (see comment)	Attrition bias possible (24%). Does not explicitly state which analyses were used.12 3.
Mijatović, 2013 [26]	Yes	Cannot tell	Cannot tell	Cannot tell	Cannot tell	Yes	Yes	Does not mention sampling method. Patients in one Serbian drug treatment centre; excluded patients with severe physical disease, mental disorders and polysubstance dependence. Unclear how side-effects (besides QTc) were measured.
Mijatović, 2017 [27]	Yes	Yes	Cannot tell	Cannot tell	Yes	No	Yes	30 patients enrolled in study; 17 patients present six months after starting MMT.