The 2005–2012 Bangkok Tenofovir Study, a randomized, double-blind, placebo-controlled study, examined participants' adherence to daily oral tenofovir in an HIV pre-exposure prophylaxis (PrEP) trial; identified factors associated with adherence; and assessed the impact of adherence on the risk of HIV infection among people who inject drugs (PWID). The study took place in 17 Bangkok Metropolitan Administration drug treatment facilities that offer an HIV-prevention package, social services, and medical care. The 2,413 participants attended either daily or monthly visits during which nurses observed participants swallowing the study drug, and participants from both groups initialed a diary, which the authors used to assess adherence. Higher levels of adherence were associated with reduced risk of HIV infection (83.5 percent among participants with at least 97.5 percent adherence, compared to a 48.9 percent reduction overall). Analysis showed better adherence among participants aged 40 years and over, and among women. Participants who had been incarcerated or had injected methamphetamine before enrollment were more likely to report below 95 percent adherence, suggesting poor adherence among some at-risk participants. These findings were consistent with findings from trials among men who have sex with men and HIV-discordant heterosexual couples. The authors suggested that PrEP could provide high levels of protection against HIV for PWID, provided adherence is high.