Combination HRT drugs which add progesterone fail to protect against endometrial cancers atcertain doses.

This new finding undermines the rationale for adding progestogens to HRT preparations, which was meant to protect against endometrial cancer.

In this new study, the risk was quadrupled among those who took unopposed estrogen. Nevertheless, the overall risk remained at 1.4 times among those who took a combined therapy of estrogen with cyclic progestogen.

Furthermore, progesterone needs to be taken for up to 21 days a month before it has any appreciable effect. If it is taken for 10 or fewer days, the cancer risk is still high at around 3.1 times, compared to someone not taking HRT.

And the benefits seem to reduce over time. After five years, those who had taken progestogen for fewer than 10 days a month had an increased risk of 3.7 times and even those who took it for up to 21 days still had a 2.5 times greater chance of developing endometrial cancer.

These findings are based on a trial among 1,154 postmenopausal women in the state of Washington, and they contradict earlier studies that had concluded that the combined hormone therapy actually offered protection against cancer (The Lancet, 1997; 349: 458-61).