This is from here. www.menopausematters.co.uk/risks.php
Dr Heather Curry is the author- NHS gynaecologist
NOTE that the stats of the risks do not include women using micronised progesterone. There is very little data on that and the available observational stats show a lower risk compared to other progestogens.
Risks of HRT
Risks associated with HRT include association with increased risks of breast cancer (with long duration HRT), blood clot and, if HRT is started many years after the menopause, possibly cardiovascular disease.
For the majority of women who use HRT under the age of 60, and for many beyond that age, the benefits of HRT outweigh any risks but for some, alternative treatments for either symptom control or osteoporosis prevention/treatment may be recommended or preferred.
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 an association with a small increased risk. Once HRT has been stopped, the risk returns back to baseline. This suggests that the use of HRT may promote the growth of breast cancer cells which are already present if HRT is taken for more than 5 years after the age of 50, in some women; there is no evidence that HRT causes breast cancer.
It seems very likely that different types of HRT are associated with different risk, estrogen appears to increase the risk very little while there appears to be a small increased risk of cancer promotion with long term use of estrogen combined with progestogen (combined HRT).
The NICE guideline on Diagnosis and Mangement of Menopause concluded that for every 1000 women aged 50 to 59, combined HRT may be associated with an extra 5 cases of breast cancer over 7.5 years, with no extra cases for women taking estrogen only. (NICE guideline)