The HIV epidemic in Estonia affects the population of people who inject drugs (PWID) the most, but factors associated with adherence to antiretroviral therapy (ART) among PWID have not been thoroughly examined in Estonia, with particularly limited data regarding beliefs and attitudes of PWID. The objective of this study was to explore the association between ART adherence and individual beliefs, perceived effectiveness of ART, and self-rated health in particular, in this specific population. The study used baseline survey data from a longitudinal intervention study of HIV prevention among PWID in Estonia, in which 107 HIV-infected participants reported current use of ART. Current adherence was measured through the use of a visual analog scale. Approximately half (49%) of the participants reported optimal (≥95%) adherence. The vast majority (81%) believed in the effectiveness of ART. Less than a quarter of the participants (22%) rated their health as good or very good, and a half (52%) reported average health. Individual beliefs and self-reported health were not associated with ART adherence in both bivariate and multivariable analyses. Participants with problem drinking reported significant suboptimal adherence to ART (adjusted odds ratio [AOR] 0.42, 95% CI 0.19–0.97). Daily injection drug use was also associated with suboptimal adherence (AOR 0.34, 95% CI 0.13–0.91). Problem drinking has not been commonly reported as a factor of suboptimal ART adherence among PWID; further research would be useful to identify the pathways that might be involved.