## 1 Introduction
### Application of SDT in the Context of Drug Use and Drug Relapse  
According to SDT, autonomous self-regulation (i.e., motivation that is intrinsic, identified, or integrated) is important for promoting healthy behavior because it helps one engage in performing such behavior and in making the behavior sustainable [27]. To help enhance one's motivation to make positive changes, both the personality (i.e., individual level) and the context (i.e., social/environmental level) are essential. At the individual level, for example, people can be encouraged to explore the environment autonomously so as to foster their autonomous self-regulation [26]. At the social/environmental level, autonomy support/need support, which \"involves encouraging others to be self-initiating rather than pressuring them to behave in a particular way,\" is important (e.g., the practitioner taking the perspective of the service users and understanding their feelings, giving them information support and emotional support) [32] (p. 1114). The sole existence of the individual level is insufficient. Autonomy support/need support must exist in the social/environmental context (i.e., one must perceive that the three basic, innate psychological needs are satisfied) to encourage healthy development and well-being, or else one's development of autonomous self-regulation and the internalization process will be blocked [25; 26; 32]. Applying SDT to the context of drug abuse and drug relapse, it is postulated that solely providing external motivators (e.g., parental pressure) is not sufficient for one to quit drugs, because one may engage in treatment out of fear of any possible consequences, but not because of an internal sense of control and autonomy [33]. Instead, building good relationships with the drug users and offering them autonomy support/need support (e.g., respecting their choices with respect to treatment; providing a safe environment for them to practice adaptive behavior; offering them acceptance, warmth, understanding, support, and unconditional positive regard) may help facilitate their internalization and autonomous self-regulation. This helps foster their attainment of the goal to quit taking drugs based on intrinsic motivation [25; 34]. According to [35], service users of a methadone maintenance program were more engaged in the program and experienced lower occurrence of drug relapse when they were highly intrinsically and autonomously motivated. While both high internal and external motivations predicted optimal treatment outcomes, the sole existence of external motivation negatively predicted adherence to the program, reflecting that coercion or external forceful pressure is not useful in helping someone to stop taking drugs. Summarizing the above literature, it is known that intrinsic motivation is a fundamental incentive driving individuals to direct behaviors. The same applies to drug users. To nurture their intrinsic motivation, the provision of social context is central. Such context should provide support for their autonomy, competence, and relatedness so as to encourage them to move towards healthy development and well-being [25; 26]. For example, when they perceive that: (1) their viewpoints, choices, feelings, and emotions are understood, acknowledged, and supported without judgment (e.g., their difficulties in dealing with stress and the drug cessation process are understood by another person); (2) the advice given by others is backed up by an underlying rationale rather than exerted in the form of control and pressure; (3) there are a number of effective strategies available for making changes; (4) their positive changes (e.g., giving up drugs) are relevant to their personal aspirations in life; (5) they can succeed in making positive changes (e.g., giving them unconditional regard; helping them understand that failures pave the way to future successes); and (6) the plans for quitting drugs are appropriately designed to fit their needs and level [26].