From the Cancerresearchuk.org website
HRT and cancer
The evidence that HRT can cause some types of cancer (breast, womb and ovarian) is strong. It includes a study of one million women, run by Cancer Research UK scientists, which has shown that different types of HRT can increase the risk of different cancers.
Breast cancer
The risk of developing breast cancer is increased for women who are using combined HRT, and may also be increased for users of oestrogen-only HRT. The Million Women Study results suggested that women who are using combined HRT have double the risk of breast cancer compared to non-users. And if they use HRT for over 10 years, their risks are even higher.
Once a woman stops using HRT, her risk of breast cancer will start to drop back down again. It takes about 5 years to go back to the same risk as if she hadn't used HRT.
Breast cancer is much more common than either ovarian or womb cancer. This means that, if you look at the total effect of HRT on these types of cancer, combined HRT is linked to more cases than oestrogen-only HRT, because of its effect on the risk of breast cancer.
Womb (endometrial) cancer
HRT which uses oestrogen-only increases the risk of womb cancer. But the picture for combined HRT is a bit more complicated. There is evidence that the progestogen part of combined HRT can counteract the cancer-causing effects of the oestrogen part. Studies have shown that the more days per month a woman uses progestogen, as part of combined HRT, the less the increase in risk of womb cancer. At the moment there isn't enough research to say whether, above a certain dose, the progestogen can outweigh the oestrogen and actually reduce the risk of womb cancer overall.
Ovarian cancer
Using either oestrogen-only or combined (oestrogen and progestogen) HRT increases a woman's risk of ovarian cancer. In 2015 new research found that there is an increased risk of ovarian cancer even for women who are taking HRT for under 5 years.
When a woman stops taking HRT the risk starts to go back down over time.
In the UK, for every 1000 women using any type of HRT for 5 years from age 50 there is thought to be 1 extra case of ovarian cancer.
Other cancers