ABSTRACT Chemsex, the use of psychoactive drugs before or during sexual activity, is a growing public health concern due to its association with increased HIV transmission and other harms, particularly among men who have sex with men (MSM). Current estimates suggest that 9-26% of Malaysian MSM participate in chemsex, contributing to recent increases in HIV prevalence. In the absence of evidence-based interventions for chemsex, harm reduction strategies remain the most impactful approach to mitigating chemsex-associated harms. However, getting harm reduction interventions to MSM who engage in chemsex is a major challenge due to the spontaneous and dynamic nature of chemsex risk. Just-in-time adaptive interventions (JITAIs) delivered via smartphones represent a powerful strategy to deliver support by deploying tailored intervention when needed. Specifically, apps that incorporate JITAI can be more effective than traditional app-based interventions by addressing the dynamic nature of chemsex risk and capitalizing on users' changing states of vulnerability (heightened chemsex risk behaviors) and receptivity (willingness to engage in intervention), while also minimizing user burden, disruption, and habituation. Although JITAIs are increasingly being used in domains such as addiction, mental health disorders, physical inactivity, and obesity, research on JITAIs to address chemsex harm reduction is non-existent. In response, we developed JomCare, a smartphone app-based chemsex harm reduction JITAI, that uses a machine learning algorithm to determine risk and deliver tailored support as needed. JomCare includes several `pull' and `push' intervention components based on the information-motivation-behavioral skills (IMB) model, and has demonstrated high feasibility and utility in our recent pilot work. However, little is known to guide which intervention components should be delivered in specific contexts to achieve maximum benefit, thus indicating the need to optimize JomCare. Framed by the multiphase optimization strategy (MOST) and building on our formative work, we will optimize JomCare using a micro-randomized trial (MRT) to evaluate: i) which theory-driven intervention components are efficacious in reducing chemsex risk behaviors; and ii) which contexts influence the efficacy of JomCare. Specifically, we will conduct a 90-day MRT of the JomCare JITAI among 482 chemsex-involved Malaysian MSM. Participants will be randomized twice daily via the app to receive: i) no prompt; ii) a generic engagement prompt; or iii) one of three IMB model-based engagement prompts. The specific aims of this application include: i) Evaluate the effects of any intervention (i.e., theory-driven or generic engagement prompts) versus no intervention on chemsex risk behaviors (proximal outcomes) at subsequent EMAs following randomization; ii) Compare the effects of theory-driven and generic engagement prompts on proximal outcomes; and iii) Examine contextual moderators of intervention on proximal outcomes.

NARRATIVE Chemsex, the use of psychoactive drugs before or during sexual activity, is a growing public health concern associated with striking increases in HIV, particularly among men who have sex with men (MSM). This study proposes to optimize an app-based just-in-time adaptive intervention, called JomCare, which aims to reduce chemsex risk by delivering tailored support when a person is most likely to be vulnerable to chemsex risk behaviors and receptive to intervention, with the purpose of reducing risk and averting new HIV infections.