New evidence from the Netherlands shows that long term tamoxifen use increases the risk of endometrial cancers with a poor prognosis.
Dutch researchers reached this conclusion after reviewing the cases of 309 women who developed endometrial cancer after treatment for breast cancer and comparing these to 860 controls with breast cancer only.
They found that tamoxifen use increased the relative risk of developing endometrial cancer by 1.5 overall compared with non users. The relative risk increased to 2.0 for women who took tamoxifen for two to five years, and to 6.9 in those who took the drug for at least five years (Lancet, 2000; 356: 868-9, 881-7).
In addition, the endometrial cancer which developed was harder to treat and more likely to be the cause of death among tamoxifen users.
These findings contradict previous, smaller studies suggesting that tamoxifen is associated with low grade endometrial cancers with a favourable prognosis. They also call into question the widespread use of tamoxifen as a prophylactic against breast cancer in healthy women.
New evidence suggests that screening tests are useless at detecting endometrial cancer in women taking tamoxifen. These tests, often in the form of painful biopsies and invasive transvaginal ultrasound examinations of the uterine lining, have been shown to be no more effective at diagnosing early endometrial cancer than watching for signs of abnormal uterine bleeding (Journal Watch, 2000; 20: 164).