Women at increased risk of breast cancer taking tamoxifen prophylactically were more likely to be non-adherent if they experienced adverse effects in the first 6 months of therapy, according to a study published in the Journal of Clinical Oncology.1

Patients see at least a 30% reduction in cancer incidence using preventative therapy, yet only 16% of women start once offered. A significant challenge to initiation and adherence is patient concern with adverse effects. In this study, authors investigated the effect of adverse events on long-term adherence.

The International Breast Cancer Invention Study (IBIS-I) was a randomized controlled trial that enrolled pre- and post-menopausal women deemed to be at increased risk of breast cancer. Patients were assigned 1:1 to receive tamoxifen or placebo for 5 years.

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Significant differences in rates of adherence occurred after 12 months, and were most prominent at 54 months. The initial 12 to 18 months had the highest rates of discontinuation but decreased afterwards.

Patients experiencing nausea/vomiting were the most likely to be non-adherent — tamoxifen (odds ratio [OR], 0.57; 95% CI: 0.37-0.86; P = .007) and placebo (OR, 0.58; 95% CI: 0.37-0.93; P = .023) — in both study arms.

Gynecologic symptoms were significant only in the tamoxifen arm (OR, 0.77; 95% CI, 0.62-0.97; P = .024).

The overall rate of adherence for at least 4.5 years was about 70% (tamoxifen: 65.2% vs placebo: 74.0%; P < .001).

The treatment arms had similar rates of non-adherence, suggesting that patients may misattribute naturally occurring symptoms to preventative therapy.

RELATED: Tamoxifen or Exemestane With Ovarian Function Suppression Prolongs Breast Cancer-free Interval

The study authors concluded that “intervention strategies are needed to promote adherence, as well as to effectively communicate the harms and benefits of preventative therapy to participants.”


  1. Smith SG, Sestak I, Howell A, Forbes J, Cuzick J. Participant-reported symptoms and their effect on long-term adherence in the international breast cancer intervention study I (IBIS I). J Clin Oncol. 2017 Jun 29. doi: 10.1200/JCO.2016.71.7439 [Epub ahead of print]