Knowledge of potential harms and benefits of tamoxifen among women considering breast cancer preventive therapy

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 judgements 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. 258 (63.2%) completed a 3-month follow-up survey. 15.7% of participants recognised 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%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 recognised all harms and benefits (vs. not recognising) (OR=2.47, 95%CI: 0.94-6.54). 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.