Dutch researchers concluded that long term use of tamoxifen by women with breast cancer increased the risk of developing endometrial cancer by 50%. People taking the drug for 2-5 years ran a higher risk (double). People taking it for five years or more ran a sevenfold increased risk.

Earlier, smaller studies linked tamoxifen with low-grade, relatively treatable endometrial cancer. This study found that the cancers were harder to treat and more likely to be fatal.