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.

This study will develop a dataset of annotated smartphone usage in the context of PrEP adherence and nonadherence. In order to understand smartphone context, the investigators will utilize a digital pill system (DPS) to measure real-time PrEP ingestion events. Eligibility will be confirmed during the Screening Visit (Visit 1). During the Enrollment Visit (Visit 2), participants will complete a baseline quantitative assessment (e.g., sociodemographics, sexual history, attitudes about technology), receive training on the operation of the DPS and Beiwe, and will ingest their first digital pill dose under observation by study staff. Participants will be instructed to take one PrEP pill once daily while using the DPS and Beiwe app for 60 days. Participants will return for a study visit at the end of month one (Visit 3) for a refill of digital pills. Following a 24-hour period which no PrEP ingestion is detected by the DPS, participants will receive a brief online substance use and sexual risk survey, which will inform specific contexts of nonadherence. At the Month 1 Visit (Visit 3) and Month 2 Visit (Visit 4), timeline followback discussions will be conducted to better understand the context of any PrEP nonadherence events detected by the DPS, and phenotypic data collected by the Beiwe app, during the prior 30 days. At the Month 2 (Visit 4), the investigators will administer a blood draw for dried blood spot (DBS) testing to measure participants' PrEP adherence and compare it against adherence detected by the DPS. To gain insight into user experiences operating the DPS and Beiwe app as part of the study, the investigators will conduct the semi-structured qualitative exit interviews with participants at Month 2 (Visit 4).

Inclusion Criteria:

1. Age 18 or older
2. Cisgender male
3. Has sex with men
4. HIV negative
5. On PrEP or initiating PrEP (including switching from 2-1-1 or "on-demand" PrEP dosing to once-daily dosing)
6. Moderate risk score on ASSIST substance use screener (11-26 for alcohol, 4-26 for all other substances) or higher
7. Qualifying laboratory testing for PrEP: Cr clearance in past 3 months, HBV vaccination, liver function tests
8. Owns a smartphone with Android or iOS

Exclusion Criteria:

1. Does not speak English
2. History of Crohn's disease or ulcerative colitis
3. History of gastric bypass or bowel stricture
4. History of GI malignancy or radiation to abdomen
5. Allergy to gelatin, silver, or zinc (components of digital pill)
6. Allergy to PrEP
7. Not willing to operate DPS or Beiwe app

Cisgender men who have sex with men