Million Women Study compares combination HT to estrogen therapy

While combination estrogen/progestagen therapy decreases the risk of endometrial cancer compared with estrogen therapy alone, it causes a greater increase in breast cancer and, thus, a greater increase in total cancer incidence when used either continuously or cyclically, according to the latest findings from the Million Women Study.

Researchers recruited over 700,000 postmenopausal women from the UK without previous cancer or hysterectomy and followed them for an average of 3.4 years. They found that compared with those who never used HT, the risk of endometrial cancer was reduced in women who last used a continuous combined HRT preparation (relative risk 0.71); increased in women who last used tibolone (RR 1.79), and in women who last used estrogen only (RR 1.45); and did not significantly change in women who last used a cyclic combined preparation (RR 1.05).

In addition, the researchers found that the adverse effects of tibolone and estrogen-only HRT were greatest in non-obese women and that the beneficial effects of combined HRT were greatest in obese women.

Million Women Study Collaborators. Endometrial cancer and hormone-replacement therapy in the Million Women Study. Lancet. 2005;365:1543-1551.

Brinton LA, Lacey JV Jr, Trimble EL. Hormones and endometrial cancer-new data from the Million Women Study. Lancet. 2005;365:1517-1518.