Background: Effective communication is essential to build trust and improve patient understanding of cancer diagnosis and treatment. Little is known about patient comprehension of provider explanations and whether better communication influences adherence to recommended treatment especially in breast cancer (BC) which may require years of adjuvant treatment. Methods: Between 7/2023-8/2023, active patients and survivors were recruited from Breastcancer.org and consented to participate in an anonymous online survey in English or Spanish. Eligibility included: US resident, age ≥18, and diagnosed with BC within the past 10 years. Survey assessed patient-reported overall communication, side effect communication, and treatment adherence. Multivariate analysis (MVA) assessed relationship of patient-reported understanding and treatment adherence after controlling for gender, age, stage, race, ethnicity, and education. Results: Of 1266 screened, 997 met eligibility and participated. 98.2% identified as women, 81.2% White, 4.8% Black, and 4.2% Hispanic. Median age was 62 (interquartile range IQR 53-69) and median time since diagnosis 2.1 years (IQR 1-5.1). 6.5% had metastatic disease. 77.3% had a college degree. 93.4% participants reported their doctors “usually” or “always” explained things in a way they could understand. On MVA, there was a trend for minoritized groups (Black, Hispanic, Indigenous patients) to report their doctors were less likely to explain things well (OR 0.57, 95%CI 0.30-1.06, p=0.08) but there were no significant differences. Most felt their doctors “definitely” (45.8%) or “to some extent” (10.3%) explained possible short-term side effects. One-third (35.2%) stated their doctors explained “to some extent” and that they also did their own research to fully understand short-term side effects; 6.1% stated that their own research was the only thing that helped them understand as their doctors did not explain. Less than one-third (27.9%) felt their doctors “definitely” explained possible long-term side effects. 7.7% participants took less medication that their doctors prescribed due to treatment side effects, 15.1% stopped taking a medication or treatment altogether. Patients who reported that their doctors were “sometimes” or “never” understandable were more likely to reduce or stop their treatment (44.4% vs 21.3%, p<0.001). This held true on MVA: patients reporting poor communication were less likely to adhere to recommended treatment (OR 0.36, 95%CI 0.21-0.62, p<0.001). Conclusions: Surveyed patients with BC report good provider communication skills overall but note that side effect profiles are not always well-explained, especially for long-term side effects. More than 1 in 5 did not follow some aspect of BC treatment as recommended, and those who reported poor communication with their providers were 64% less likely to be adherent to recommended care.