Introduction Drug abuse is a worldwide problem especially among youth. Due to easy access to a wide range of illicit drugs via multiple approaches in Hong Kong, growing drug abuse is exposing and spreading among the youth. According to the Central Registry of Drug Abuse Report in Hong Kong, 87.3% of drug abusers reported the first abuse of drugs at ages of 25 or below. The comparison of the Central Registry of Drug Abuse between 2014/15 and 2017/18 showed that the number of students claiming to have used drugs has gone up by 23 percent compared to the last survey, and 81% of them started to use drugs at an age younger than 15-year. The early age of first drug use is strongly associated with the risk of developing a substance use disorder in later life. Youth who persistently abuse substances often experience an array of problems, including academic difficulties, health-related problems (including mental health), poor peer relationships, risky sexual behavior, the likelihood of having blood-borne diseases, and involvement with the juvenile justice system. Additionally, there are harmful consequences for family members, the community, and the entire society. The statistics above alarm that youth drug abuse in Hong Kong is becoming a major problem and can not be overlooked.

The combination of medical treatment and counselling is the most common treatment for drug abusers. However, more than half of the youth drug abusers reported never attempting to quit, only 12.2% of youth drug abusers attempted to seek help, and more than 75% did not accept medical treatment as they did not think they were addicted. Although many non-pharmaceutical interventions have been applied, a meta-analysis found that the existing non-pharmaceutical interventions showed insignificant effectiveness in helping youth drug abusers to change their behaviour. One of the major reasons for the failure to assist youth drug abusers to change was that the patient education method may lead the youth abusers resistant to accept the intervention. Previous research has confirmed that the recovery from drug abuse is influenced by the users' intention to change, which is determined directly by their perceptions of anti-drug and their perceived self-efficacy to change. While, drug-abusers' attitudes, social influences, and demographic characteristics have more indirect effects on recovery from drug abuse. Another reason may be that standard interventional content may not be able to meet the personal needs of drug abusers. In addition, the meta-analysis also showed that individual interventions with more than one session had a stronger effect, while previous brief interventions provided insufficient sessions to support the behavior changes among youth abusers. In addition, according to qualitative interviews conducted for youth drug abusers, adolescents were reluctant to seek help from sources that they believed would be judgmental, lacked expertise, personal information exposed or would inform their parents. Given that, more feasible, individualized, and effective non-pharmaceutical interventions should be developed and evaluated to help this youth population to change.

Conceptual framework Medical peer-delivered counselling Comparing with general counselors, the use of peer counselors can increase the appeal of addiction services to youth and young adult users. Since most young people are not eligible to hold positions of power in their society, they find themselves, subject to authority. Such powers differential made communication between adults and youth difficult. When one is trying to engage the youth in drug abuse reduction initiatives, this power differential manifests itself also as a knowledge gap. Comparing with adults, the equality in power status between the youth makes peer-delivered risk communication successful. However, Addiction counseling on drug abuse is more complex compared with smoking cessation or alcohol and requires more professionalism. With the advantages of similar age and capability to better master related knowledge and counseling skills, peers with a medical background can address youths' concerns relevant to their experience.

Instant messaging Given the ubiquity of mobile technology among youth, and the considerable logistical barriers to face-to-face-based care, instant messaging (IM) via mobile applications such as WhatsApp or WeChat is increasingly used as a new strategy for health promotion and to enhance treatment compliance. As reported in the previous study, drug-abusing clients prefer to use online counseling as it was less confronting to traditional forms of counseling by providing a private and emotionally safe environment. Therefore, the interaction via smart phone apps encourages those who are reluctant to get help face-to-face to seek direct anti-drug advice. In addition, another advantage of using instant messaging is to offer quick, real-time interactions, thus delivering continuous professional advice and personalised support to drug abusers to help them overcome withdrawal symptoms.

Brief Motivational Interviewing Motivational interviewing (MI) was originally developed in the field of alcohol abuse and found to be transferable to other health-related behaviors. Differing from prevailing patient education methods, MI is a directive, client-centered counselling strategy that encourages clients to explore and resolve their ambivalence and promotes their confidence in their ability to change their behaviour. Brief MI, with the advantage of manpower saving, shares the same core as MI that individuals are advocates to initiate and continue behavioural change, and yet often in a state of ambivalence with fluctuating motivations before the behavioural change. Brief MI emphasizes adopting shorter and simpler strategies, which include opening strategy, a typical day, the good things and the less good things, providing information, the future and the present, exploring concerns, and helping with decision-making. By using these specific techniques in the dynamic and interactive process, brief MI explore and resolve the ambivalence, develop discrepancies between individuals' core belief and the behaviour of not engaging in desirable health-related lifestyle practice, consequently enhancing the confidence and motivation in the behavioural change.

Incorporating of Medical peer counselling, instant messaging, and brief MI in promoting drug abuse reduction among youth In combination, the medical peer-delivered intervention via instant messaging interaction may be more effective in helping drug-abusing youth to eliminate or reduce the fear of personal information exposure, the unknown to expect from the treatment, and the whole lifestyle change, and worries about withdrawal symptoms. This approach has the potential to increase the help-seeking behavior and intention to quit among youth drug abusers who reluctant to seek help from services requiring real-name registration. By using the brief MI strategies, youth drug abusers can be guided to attempt behaviour change step by step, and subsequently, could abstain from drugs with assistance.

As stated above, the proposed intervention may be feasible and own potentially effective in reducing drug abuse among youth. However, according to our literature search, there is a paucity of studies exploring its effectiveness. Given that, this study aims to conduct a randomized controlled trial to inform the effectiveness of a medical peer-delivered intervention of interactive brief motivational interviewing via instant messaging communication in reducing drug abuse among youth in Hong Kong