To our knowledge, this scoping review is the first to undertake a systematic synthesis of PCC in substance use disorder treatment settings. To strengthen the breadth and specificity of this review, existing frameworks of PCC from other disciplines were used to guide the search strategy and data charting methods [31], and the directed content analysis allowed population and context specific nuances to be identified. The findings suggested that few references had examined all four principles of PCC, although 42% described more than one PCC principle. The most frequent principle identified was therapeutic alliance and the most frequent outcomes measured included substance use and treatment engagement. The findings contribute evidence that can be used to support a comprehensive and evidence-based conceptualization of PCC with implications for its implementation and evaluation.

The first objective was to determine which PCC principles have been described in substance use disorder treatment settings, and the results revealed that therapeutic alliance was the most frequently described principle. The first plausible explanation for this is the longstanding tradition of therapeutic alliance in psychotherapeutic research and practice [100]. In the present review, two-thirds of the references offered primarily psychosocial treatments (e.g., cognitive behavioral therapy) for substance-related disorders. In this discipline, therapeutic alliance receives significant attention given its importance in predicting counseling outcomes [32, 100]. In the references that described therapeutic alliance, over 50% were empirical quantitative papers and conceptualized therapeutic alliance according to client, provider, or observer-rated empirical measures, such as the Working Alliance Inventory (WAI). Thus, this longstanding tradition to examine the extent of therapeutic alliance likely contributed to the high number of references in the present review that described this PCC-principle.

Our search also yielded references that delivered additional treatments (e.g., pharmacological treatments alone or combined with psychosocial) in alternative settings (e.g., residential detoxification programs, harm reduction services), which provided an opportunity to determine that non-judgment, respect, empathy, and understanding were also common characteristics of therapeutic alliance. While respect has been described in broader conceptual analyses of PCC [29, 101], information regarding why these attributes were important among people with substance use strengthens its interpretation in this context. Examples of these reasons included clients’ safety [79, 102]; to gain clients’ trust [45, 63, 103]; and to reduce stigma [63, 96, 104]. These defining attributes are especially salient when considering that experiences of stigma are common among people with substance-related disorders [105,106,107] and have been identified as barriers to treatment [12, 14].

These relational characteristics also intersected with shared decision-making, such that the analysis of antecedents to PCC revealed that respectful and understanding relationships promoted shared decision-making. The reciprocal was also found, whereby collaborative approaches strengthened therapeutic alliance. These antecedents give more depth to our finding that the defining characteristics of shared decision-making denoted an underlying philosophy of respect towards clients as “integral … rather than passive” partners in the treatment process. The first defining characteristic emphasized a joint decision-making process. This category primarily described a process of dialogue and discussion that granted clients a more active role in the decision-making process and facilitated the health care provider’s understanding of clients’ needs and expectations. This view of shared decision-making resembles those of the broader PCC-frameworks that have conceptualized this principle as “sharing power and responsibility” [28], “finding common ground” [108] and also more recent proposals for the clinical practice of shared decision-making [109]. However, the second defining characteristic emphasized a fully autonomous decision-making process, which is more closely aligned with other frameworks’ notion of “empowering care” [29, 110]. Those existing frameworks describe autonomy, self-confidence and self-determination as core characteristics of this principle. While, there is evidence that increasingly recognizes clients’ preferences to be more actively involved in substance use disorder treatment decision-making [22], further work might explore the circumstances under which a deliberative or autonomous decision-making process is more suitable from the client and health care provider’s perspective.

The integration of shared decision-making practices often presumed an individualized care approach, such that the process of dialogue involved discussion of clients’ unique needs, circumstances, traditions and preferences [29]. One study to highlight is that of Joosten et al. who developed and tested the effectiveness of a shared decision-making intervention in an inpatient treatment setting [73, 80, 81]. Their intervention relied on an individualized assessment of clients’ needs and goals (via the Camberwell Assessment of Need). The client and clinician then discussed their independent ranking of the priority of these goals and adapted treatment accordingly. In this review, individualized care did not always include shared decision-making however; it was also described by several treatment matching approaches, such as as-needed-dosing [40, 46, 90]. Regardless of the specific design chosen, these findings imply that comprehensive assessments and flexibility in service design and delivery (at a clinical and organizational level) supported individualized care.

Thus, individualized needs assessment and treatment delivery overlapped with holistic, trauma-informed and culturally competent, responsive, and appropriate care with respect to their common goal to provide comprehensive and flexible care, adapted to client-identified needs and values. Under ideal circumstances, such consideration would be facilitated by an assessment of clients’ bio-psycho-social needs [47], which are often inextricable from their cultural context and the pervasive impacts of structural and interpersonal trauma [20]. In the present review, the defining characteristics of these principles included specific practices adopted by health care providers (e.g., comprehensive needs assessments, avoiding re-traumatization). However, the inductive analysis of antecedents to PCC revealed that both the system (e.g., a vision of shared governance; safety and stability of treatment setting; flexibility of service provision) and the health care provider (e.g., communication style) play a conjoint role in the successful implementation of PCC principles. For instance, a physician’s endeavor to adopt shared decision-making practices in the prescription of opioid substitution treatment will require a health care system that has implemented evidence-based treatment options and policies that support client-provider collaboration (e.g., flexibility around dosing schedules, frequency of visits, etc). Thus, a good starting point for moving PCC into the realm of evidence-based practice in substance use disorder treatment is a consideration of potential barriers to its implementation from the client, provider and system’s perspectives [111].

While factors supporting PCC have been relatively consistent across broader concept analyses [29, 30, 112], there has been less agreement on what consequences or outcomes of PCC can be expected and thus, measured. Examples of such outcomes have included consultation processes (e.g., communication skills, quality of care, treatment satisfaction) [29, 113], health behaviours (e.g., service utilization, adherence to treatment plans) [113, 114], health outcomes [29] and patient-reported outcomes [112]. It has also been proposed that some of these outcomes are likely more intermediate (i.e., perceived quality of care, satisfaction, consultation process outcomes), while others more distal (i.e., health behaviours and health outcomes) [113].

In the present review, substance use and treatment engagement outcomes were the most frequently investigated, regardless of PCC principle. This might have been influenced by the high frequency of references exploring therapeutic alliance, half of which related the WAI with the number of days of substance use or number of counseling sessions. However, it might also reflect a common assumption that the goal of any substance use disorder treatment is to reduce the severity of use [115]. A continued emphasis on substance use outcomes neglects that the stated goal of PCC is to improve the treatment process [26, 27]. It is also not congruent with prior research demonstrating that clients’ goals extend to other domains (e.g., health, housing, family relationships) [116] and emerging recommendations to integrate patient-centered or patient-reported measures in substance use disorder treatment [117]. Thus, future PCC research in substance use disorder treatment will be strengthened through choices of measures that reflect these goals.

This review was a necessary first step towards conceptualizing PCC for substance use disorder treatment. Although scoping reviews typically take a broad framing of the population, concept and context [33, 34], this resulted in a high number of false positives and posed several challenges to the synthesis and to teasing apart potential differences in treatment type and setting. While efforts were made to overcome these challenges (i.e., substantial resources were devoted to reaching adequate inter-rater agreement and carrying out the directed content analysis), there are further limitations to bear in mind. First, we were unable to carry out a comprehensive grey literature search in international search databases, other than TRIP, and thus, might not have adequately captured grey literature reports of the implementation of PCC in settings outside of North America. In addition, our search strategy was developed in English and eligibility was limited to references published in English, French, Spanish, Italian or Portuguese. This might have influenced the comprehensiveness and international breadth of the search and thus, inflating the number of references from North America.