Tamoxifen reduces breast cancer incidence in women at increased risk, but may cause side effects. We examined women's knowledge of tamoxifen's potential harms and benefits, and the extent to which knowledge reflects subjective judgments of awareness and decision quality. After a hospital appointment, 408 (55.7%) women at increased risk of breast cancer completed a survey assessing objective knowledge about the potential benefit (risk reduction) and harms (endometrial cancer, thromboembolic events, and menopausal side effects) of tamoxifen, and subjective tamoxifen knowledge and decisional quality. Two hundred fifty-eight (63.2%) completed a 3-month follow-up survey. Sixteen percent (15.7%) of participants recognized the potential benefit and three major harms of using tamoxifen. These women were more likely to have degree-level education [vs. below degree level; OR, 2.24; 95% confidence interval (CI), 1.11-4.55] and good numeracy (vs. poor numeracy; OR, 5.91; 95% CI, 1.33-26.19). Tamoxifen uptake was higher in women who recognized all harms and benefits (vs. not recognizing; OR, 2.47; 95% CI, 0.94-6.54). Sixty-six percent (65.8%) of tamoxifen users were unaware of its potential benefit and harms. Most (87.1%) women reported feeling informed about tamoxifen, and subjective decisional quality was high [Mean (SD), 17.03 (1.87), out of 18]. Knowledge regarding the potential harms and benefit of tamoxifen is low in women considering prevention therapy, and they may need additional support to make informed decisions about tamoxifen preventive therapy.