Background: Nutritional and dietary supplements, complementary therapies (CT: acupuncture, yoga), and alternative medicine (AM: tinctures, holistic medicines) can have high out-of-pocket (OOP) costs, driving financial toxicity. We evaluated costs associated with the use of supplements, CT, and AM in patients with breast cancer. Methods: Between 6/7-7/12/2022, Breastcancer.org members completed an anonymous online survey assessing self-identified “unexpectedly high” OOP costs encountered during care. Adjusted multivariable analysis (MVA) evaluated patient characteristics associated with high costs due to supplements, CT, and AM. Results: Among 1,710 eligible patients, 1,437 completed the survey (84% participation rate). Median age was 45 years (IQR: 35-56), and most identified as women (94%) and non-Hispanic White (NHW, 61%). One-quarter (25%) reported high OOP costs from supplements. In MVA, those with an annual income ≥$200k (OR 0.85, 95%CI 0.78-0.93, p<0.01) were less likely than those with income <$50k to report supplements as a financial burden. Those with employer-provided insurance (OR 0.92, 95% CI 0.87-0.98, p=0.02) had lower burdens than those on Medicare. Those with higher educational attainment had higher burdens than those with high school (HS) education or less (some college/post HS (OR 1.08, 95% CI 1.01-1.17, p=0.03), college grad (OR 1.15, 95% CI 1.07-1.24, p<0.01), postgraduate (OR 1.13, 95% CI 1.04-1.24, p<0.01). High OOP costs related to CT were reported by 18% of respondents. In adjusted MVA, self-identified Asian (OR 1.24, 95% CI 1.08-1.44, p<0.01) patients and those with private insurance (OR 1.08, 95% CI 1.01-1.09, p=0.02) were less burdened by CT costs compared to NHW patients and those on Medicare. High OOP costs related to AM were reported by 18% of respondents. In MVA, patients ≥46 years (OR 0.94, 95% CI 0.90-0.99, P=0.02) and those with income ≥$200k (OR 0.91, 95% CI 0.84-0.98, p<0.01) were less burdened by AM costs [compared to those ≤45 and with income <$50k]. Those on Medicaid (OR 1.11, 95% CI 1.04-1.18, p<0.01) had higher burdens than those on Medicare. Those who reported high OOP costs from AM were more likely to alter their use of prescribed medication (i.e., skip pills, not fill prescriptions, etc) to cut costs compared to those who did not report this burden (73% vs. 48%, p<0.01). Those who reported high costs from supplements were more likely to take fewer pills as a cost-cutting measure (28% vs 19%, p<0.01). Conclusions: High costs from supplements, CT, and AM pose a financial burden for up to a quarter of patients with breast cancer, and these burdens may not affect everyone equally. Downstream effects of financial toxicity from tinctures and holistic medicines specifically may decrease adherence to prescribed medications. Discussion regarding the risks, benefits, and costs of supplements, CT, and AM is an essential part of patient-centered cancer education.