One third of women taking tamoxifen for the primary prevention of breast cancer will stop taking the medication before the end of the five-year recommended period.
Many women who give up on tamoxifen cite severe side effects, such as nausea, vomiting, headaches, hot flushes, irregular bleeding, vaginal dryness and vaginal discharge. However, most of these apparent side effects of tamoxifen were actually symptoms of menopause that had been misattributed to the drug, a UK study showed.
In the study, around 3,800 women at high risk of breast cancer were randomly assigned tamoxifen (20 mg/day) or a placebo for five years.
Women taking tamoxifen reported side effects, as expected, and were more likely to stop taking the medication than the placebo group. But women taking the inactive drug who reported side effects were equally likely to not adhere to treatment, even though these symptoms could only have been due to menopause or a nocebo effect.
“Women may therefore be attributing age-related symptoms to their assigned medication,” the researchers said.
Nausea and vomiting, as well as gynaecological symptoms, significantly increased non-adherence in women taking tamoxifen.
Dropout rates for women taking the placebo were significantly higher for women who reported nausea and vomiting, or headaches. The more severe the symptoms, the more likely women were to stop taking tamoxifen or a placebo, with most women leaving the study in the first 12 to 18 months.
This research took its data from a larger international study examining the effectiveness of tamoxifen in primary prevention for women at high risk of developing breast cancer.
Doctors could use these findings to encourage more realistic expectations of the likelihood of experiencing adverse effects, particularly for women who will enter menopause during treatment, the researchers said.
Measuring oestrogen levels might also help patients accurately interpret symptoms, Cancer Council CEO Sanchia Aranda said.
“The other important thing is that we need to better support women generally to manage the side effects of menopause,” she said. Many women could manage symptoms of menopause without HRT through relaxation techniques and psychological therapy.
Tamoxifen was listed on the PBS for primary prevention of breast cancer in 2016, with up to 50,000 women a year expected to start taking the medication. In making its recommendation for PBS listing, PBAC examined a meta-analysis involving over 28,400 patients, which found that tamoxifen prevented breast cancer in 14 out of 1000 women treated.
However, tamoxifen also placed women at a slightly increased risk of endometrial cancer, deep vein thrombosis, pulmonary embolism and cataract.
Journal of Clinical Oncology 2017, online June 29