• Kirsti I Toivonen, Linda E Carlson, Joshua A Rash, and Tavis S Campbell.
• Department of Psychology, University of Calgary, Calgary, AB, Canada.
• Patient Prefer Adher. 2021 Jan 1; 15: 2039-2050.

PurposeDespite the efficacy of adjuvant endocrine therapy (AET) in reducing breast cancer recurrence and mortality, suboptimal AET adherence is common and hence an important clinical issue among breast cancer survivors. Delineating potentially modifiable patient-level factors associated with AET adherence may support the development of successful adherence-enhancing interventions.Patients And MethodsThe present study included 133 breast cancer survivors prescribed AET recruited from a cancer pharmacy. Women completed a baseline questionnaire examining psychosocial factors and self-reported adherence and consented to their prescription records being monitored for the proceeding 12 months to ascertain proportion of days covered (PDC), an objective measure of adherence. Regression analyses were used to identify the factors most strongly associated with both self-reported and objective adherence. Exploratory moderation analyses examined whether factors were differentially associated with adherence based on AET type (aromatase inhibitors or tamoxifen).ResultsAdherence was high in this sample (PDC over 12 months was 95%). Side effect severity was most strongly associated with self-reported adherence, followed by self-efficacy, and medication/healthcare system-related barriers. Medication/healthcare system-related barriers was the only factor that uniquely predicted objective adherence. Within medication/healthcare system-related barriers, fear of side effects was most strongly associated with both measures of adherence. There were no significant interactions between AET type and potentially modifiable factors in predicting self-reported or objective adherence.ConclusionSide effects, reactions to side effects, and self-efficacy may represent modifiable targets through which AET adherence can be improved. Associations between potentially modifiable factors and adherence did not vary by AET type, despite distinct side-effect profiles.© 2021 Toivonen et al.

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