This is a single-arm, observational study of HIV-negative MSM with substance use disorder. Those who meet pre-screening criteria will attend a Screening Visit (Visit 1), where the informed consent process will be conducted and study eligibility will be confirmed. Eligible participants will attend three additional visits over the course of the study - the Enrollment Visit (Visit 2), Month 1 Visit (Visit 3), and Month 2 Visit (Visit 4). Participants will take one PrEP digital pill per day, for 60 days total, while using the digital pill system (DPS) and Beiwe, a digital phenotyping app. On nonadherent days, participants will receive brief surveys prompting them to report the reasons for their missed dose, as well as their engagement in substance use and sexual activity. Timeline followback will be conducted at the Month 1 and Month 2 Visits to understand the context of any nonadherence. Qualitative user experience exit interviews and dried blood spots (DBS) will be conducted at the Month 2 Visit.