WhatsApp-based behavioral intervention and adherence to adjuvant endocrine therapy in early breast cancer: Real-world data from Brazil.

Abstract

e13706 Background: Adjuvant endocrine therapy significantly reduces recurrence and mortality in patients with early-stage estrogen receptor–positive breast cancer. Despite its proven benefit, long-term adherence remains suboptimal, mainly due to treatment-related adverse effects and unintentional non-adherence. Digital behavioral interventions may represent a scalable strategy to improve adherence in real-world settings. Methods: In this randomized study, 143 post-surgical patients with luminal breast cancer were assigned to two cohorts. Cohort A received standard adjuvant endocrine therapy (tamoxifen or anastrozole) plus weekly automated WhatsApp reminders reinforcing the importance of daily medication adherence for one year (June 2024–May 2025). Cohort B received standard of care without behavioral intervention. Adherence was assessed every three months based on patient-reported missed doses and pill counts, with remaining tablets used to estimate non-adherent days. Adherence outcomes between groups were compared using the Mann–Whitney U test, a non-parametric statistical test used to compare paired or independent samples when the data do not follow a normal distribution. Results: Most patients were women aged 45–60 years, predominantly married or partnered, with secondary-level education. Over one year of follow-up, Cohort A reported a higher number of leftover tablets compared with Cohort B. Although no statistically significant differences in adherence were observed between groups across quarterly assessments (p = 1.00, 0.67, and 0.27 for the second, third, and fourth quarters, respectively), a trend toward improved adherence over time was noted in the intervention group, suggesting that the effect of the behavioral intervention may increase with longer follow-up. Patients receiving WhatsApp reminders may have developed greater awareness of their treatment routines, resulting in more accurate reporting of missed doses, while higher non-response rates in the control group may have underestimated true non-adherence. Conclusions: In this real-world randomized study, a WhatsApp-based behavioral intervention did not achieve statistical significance in improving adherence to adjuvant endocrine therapy over one year. However, the observed trend suggests that longer follow-up and larger sample sizes may allow detection of clinically meaningful differences. These findings support continued investigation of digital behavioral strategies to optimize adherence to endocrine therapy in early breast cancer. Clinical trial information: 79420124.1.0000.5071.