Breast cancer
Current opinion is that HRT taken for less than 5 years does not significantly increase the risk of breast cancer, but studies have shown that after 5 years of use, there is a small increased risk. The relationship between HRT and breast cancer is complex and we still don’t know whether HRT causes breast cancer or speeds up its development. The longer a woman takes HRT, the greater the increase in risk of breast cancer. Women who have had breast cancer or have a strong family history should generally always avoid taking HRT.

Breast cancer now affects around 1 in 9 women in the UK. Although many more women die of heart disease and stroke, breast cancer is the disease women fear most and it occurs at a relatively younger age than heart disease or stroke. It is the main reason why most women decide not to take HRT in the long term.

Over the last few years, several large studies on HRT have been published (The Women's Health Initiative and the Million Women Study). These studies have shown that the extra risk of breast cancer varies depending on the type of HRT a woman is taking. Oestrogen-only preparations have the least effect and the greatest effect is with combined oestrogen plus progestogen preparations. Any effect of HRT on risk of breast cancer wears off once HRT is stopped and, by 5 years, is at the same level as in women who have never taken HRT.
The Women's Health Initiative Study has found that after 5 years of use, combined HRT is associated with an overall 26% increase in risk of breast cancer. The risk with oestrogen-only HRT is not significantly increased.

The Million Women Study has found that for women aged 50 who do not use HRT, about 32 in every thousand will develop breast cancer by the time they reach the age of 65 years.
If women take oestrogen only between ages 50 - 65 for 5 years, the total number of cases would be between 33 and 34 in every thousand (i.e. an extra 1-2 cases). If they take it for ten years, there would be 37 in a thousand (i.e. an extra 5 cases).

For women who take combined HRT between ages 50 - 65, the number of cases of breast cancer that would be diagnosed would be 38 in a thousand after 5 years of use (i.e. an extra 6 cases) and 51 in a thousand after 10 years of use (i.e. an extra 19 cases).

It is vitally important that all women, whether they take HRT or not, check their breasts regularly and report any changes in their breasts to their GP. After the age of 50 years, women should have mammograms performed as part of the UK National Screening Programme. Although women will stop being automatically invited to the screening programme after the age of 70 years, they can opt to continue by telephoning to make further appointments if they wish.

Venous thrombosis (blood clot in a vein)
It has been known for a long time that taking the contraceptive pill increases risk of venous thrombosis.  This can lead to a deep vein thrombosis (DVT) and the clot may travel to the lung to cause a pulmonary embolism. It is now known that HRT also increases this risk, but to a much lesser extent. Overall the risk of any healthy woman having a venous thrombosis is very small. The risk of venous thrombosis increases as women get older. In most fit and active women, the increased risk of venous thrombosis when taking HRT is not a major problem.

It is estimated that around 3 out of every 1000 healthy women aged 50 - 59 years and not taking HRT will experience a venous thrombosis over a 5 year period. There will be approximately 4 additional cases of venous thrombosis in 1000 women taking HRT, over and above the 3 events which would occur anyway. There would be around 9 extra venous thromboses per 1000 women aged 60-69 years using HRT.

Venous thrombotic events are more likely to occur in the first year of using HRT. See your doctor at once if you develop a red, swollen or painful leg or have sharp pains in your chest. Patches have been shown to have less effect on risk of venous thrombosis than HRT tablets and are preferred for use in women with risk factors for venous thrombosis.