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Background In resource-constrained settings, data regarding breast cancer patients' adherence to endocrine therapy (ET) and physicians’ prescribing practices is limited. This study aims to decrease this knowledge gap in a real-world clinical practice. Methods Premenopausal women with stage 0-III hormone-sensitive breast cancer and receiving adjuvant ET during the past 1–5 years were identified in three Mexican referral centers. Participants' self-reported ET compliance, clinicopathologic characteristics, ET-related knowledge and beliefs, experienced adverse effects, social support, and patient-physician relationships were evaluated. Physician ET prescribing practices were compared with the gold standard according to international and national guidelines to assess clinicians’ adherence to standard-of-care prescription. Results In total, 95/132 (72%) and 35/132 (27%) participants reported complete and acceptable adherence, respectively. Incomplete adherence was mainly attributed to forgetfulness, adverse effects, and unwillingness to take ET. Being employed/studying (p = 0.042), worrying about long-term ET use (p = 0.031), and experiencing >7 ET-related symptoms (p = 0.018) were associated with incomplete adherence. Guideline-endorsed regimens were prescribed in 84/132 (64%) patients, while the rest should have undergone ovarian function suppression (OFS) but instead received tamoxifen monotherapy. Conclusions Premenopausal Mexican women self-report remarkably high rates of adequate ET adherence. However, a considerable proportion misses ≥1 doses/month, usually because of forgetfulness. Notably, only 64% receive standard-of-care ET due to suboptimal prescription of OFS. Interventions that remind patients to take their ET, refine physicians’ knowledge on the importance of OFS in high-risk patients, and increase access to OFS could prove pivotal to enhance optimal ET implementation and adherence, which could translate into improved patient outcomes., Highlights • 72% of premenopausal patients had complete (100%) endocrine therapy adherence. • 27% of premenopausal patients had acceptable (80-99%) endocrine therapy adherence. • Incomplete adherence to endocrine therapy was mostly due to forgetfulness. • Physicians prescribed guideline-endorsed endocrine therapy in 64% of cases.