Project Summary/Abstract People living with HIV (PLWH) aged 18-24 disproportionately use tobacco with half of those individuals also using some form of cannabis. While the exact amount of PLWH aged 18-24 who co-use tobacco and cannabis is unknown, the amount of co-users is only expected to increase due to changing cannabis legislation and increasing availability and popularity of vaping both tobacco and cannabis products. Unfortunately, these changes have the potential to have a profound impact on the morbidity of PLWH aged 18-24. Tobacco alone is known to be detrimental to lung health and while cannabis has been shown to have similar physiologic effects on lung health as tobacco, the combined effects of tobacco and cannabis and specifically for those who are vaping are less well studied among PLWH. Furthermore, among individuals who are attempting tobacco cessation, cannabis use has been shown to lead to difficulties with tobacco cessation and even tobacco re-initiation. Use of a decisional algorithm tool that aids providers in prescribing pharmacotherapy for tobacco cessation during routine clinical care is one promising strategy for tobacco cessation. While the parent study of this supplement seeks to evaluate the effectiveness of this decisional tool in tobacco cessation among PLWH engaged in care at three clinical sites (University of Washington, University of Alabama at Birmingham, Fenway Community Health Center), the purpose of this supplemental proposal is to understand how tobacco-cannabis co-use affects tobacco cessation in PLWH aged 18-24 who were randomized to receive the decisional algorithm tool. Focus groups and in-depth interviews will be used to examine patient and provider attitudes about tobacco cessation modalities and cannabis’ effects on cessation. Additionally, a secondary analysis from specific aim 1 of the parent study will be performed on PLWH aged 18-24. Primary outcomes will include an evaluation of the feasibility and acceptability of the algorithm tool and a comparison of the point prevalence abstinence and cannabis frequency at 6 months between tobacco and cannabis co-users and tobacco-only users. Findings from this study will help inform the next iteration of the algorithm by providing specific information on PLWH aged 18-24 who co-use tobacco and cannabis.