This study is examining the usability and effectiveness of an innovative mobile app that integrates harm reduction strategies into the existing pre-exposure prophylaxis (PrEP) intake and adherence app, with an overarching goal to reduce the potentially negative consequences of problematics chemsex behavior for individuals and communities.

The practice of sexualized drug use is historical and has been noted across a diversity of genders and people of various sexual orientations. The term "chemsex" has been used to specifically describe the use of psychoactive substances in sexualized settings among gay, bisexual, and other men who have sex with men (GBMSM), particularly stimulants like methamphetamine and mephedrone, or other drugs like γ-hydroxybutyrate (GHB), γ-butyrolactone (GBL). Chemsex among GBMSM has coincided with the emergence of sexual-networking mobile apps, creating new opportunities for finding new partners with the intention to engage in chemsex.

Harm reduction strategies for GBMSM engaging in chemsex are needed because the use of these drugs can have negative consequences for the physical and mental health of the people who use them. One of the main risks associated with chemsex is the possibility of overdose. In addition to the risk of overdose, chemsex may also increase the risk of sexually transmitted infections (STIs) and HIV transmission due to the higher possibility to engage in risky sexual behaviors.

Seeking help can be difficult for GBMSM who engage in chemsex due to stigma, making the harm reduction approach equally important as clinical treatment or intervention that emphasized an abstinence approach. Harm reduction strategies recognize that drug use is a complex issue and that it is not always possible or desirable for people to stop using drugs completely. Instead, harm reduction approaches aim to reduce the harm that can result from drug use and to support people in making healthier choices.

Considering also that mobile applications are another way to provide help to hard to reach population, this study aims to develop an innovative mobile app that integrates harm reduction strategies into the existing pre-exposure prophylaxis (PrEP) intake and adherence app. PrEP is a medication that can be taken daily to reduce the risk of HIV transmission. The aim of including PrEP adherence in the self-help harm reduction strategy is to ensure that participants have access to and are using an effective HIV prevention method. Behavior regulation strategies such as providing education about the risks of drug use, overdose prevention and management, and HIV prevention through correct PrEP intake, can be integrated into a mobile app that aims to minimize the negative consequences of drug use and reduce the risks associated with it, with an overarching goal to reduce the potentially negative consequences of problematics chemsex behavior for individuals and communities.

Inclusion Criteria:

• individuals need to have a history of drug use (ecstasy, ketamine, crystal methamphetamine, mephedrone, and/or GHB/GBL)in a sexualized context in the previous year
• can read and understand chinese,
• willing to communicate with the research team through the mobile messenger app Line.

Exclusion criteria:

• None

chemsex engaging individuals aged 18 years or older that are cisgender men or transgender female who have sex with men are eligible for the study.

Twenty-five eligible participants were enrolled and completed the baseline survey, of whom twenty completed the follow-up.

From enrollment until the day of interview, up to 1 month