Abstract
Daily oral pre-exposure prophylaxis (PrEP) is highly effective for HIV prevention, though efficacy depends on adherence. Digital pill systems (DPS) can enable direct, real-time adherence measurement. HIV-negative men who have sex with men (MSM) with substance use (excluding alcohol) utilized a DPS over 90 days and completed weekly surveys reporting sexual activity, condom use, and substance use. Responses indicating (1) any sexual activity and substance use or (2) condomless anal intercourse (CAI) in the prior week were categorized as high risk for HIV acquisition. PrEP adherence data for the 7-day period preceding each response was dichotomized as ≤ 3 and ≥ 4 doses/week, indicating prevention-effective adherence, and compared by HIV risk level. Thirteen MSM were analyzed (median age: 32). Of 113 surveys, 48.7% indicated high HIV risk, with 12.4% reporting CAI alone, 16.8% any sexual activity and substance use, and 19.5% both CAI and substance use. Weekly mean PrEP adherence was 90.3% (6.3 of 7 doses/week), with ≥ 4 doses/week recorded during 92.0% of weeks. The proportion of participants with ≥ 4 recorded doses/week was 88.9% during weeks with CAI alone, 89.5% during weeks with any sexual activity and substance use, 92.0% during weeks with both CAI and substance use, and 92.8% during lower risk weeks. Participants ingested ≥ 4 doses/week during 89.1% of all high-risk weeks and 94.8% of low-risk weeks. Overall, participants maintained high levels of PrEP adherence while engaging in HIV risk behaviors. DPS can be deployed concurrently with data collection tools to assess ingestion patterns during periods of elevated risk.