When HRT was first used in the 1960s it was oestrogen-only. There is a lot of evidence now about the effect progestins have on breast cancer- ie more cases with combined HRT than oestrogen-only. There is still a debate about whether it's dose-dependent or if the type of progestin is a factor too. There is some research which shows that progesterone is less of a risk than synthetic progestins. I've included something here from one paper which incidentally raises the comparison between the risks of endometrial cancer and breast cancer.
Here we are...
an extract from the paper on the effects of different progestogens
Progestins and progesterone in hormone replacement therapy and the risk of breast cancer
Carlo Campagnoli,1,* Françoise Clavel-Chapelon,2 Rudolf Kaaks,3 Clementina Peris,1 and Franco Berrino4
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In accord with trial results, almost all observational studies published over the last 5 years have also reported an increase in BC risk associated with progestin use in HRT [11,13–20,38,39,43,44,139]. The reported increase in risk was 2–4 times greater than that associated with the use of unopposed estrogen (Tables 4 and and5).5). Furthermore, although there were exceptions [11,16,18,19], most studies providing information on the two regimens of progestin addition [14,15,17,38,39,139] found that the risk was greater with continuous-combined than sequential regimen (Table 6).
Estrogen plus progestin therapy and breast cancer: consequences of the sequential regimen (sEPT) and the continuous-combined regimen (ccEPT) (observational studies)
These findings, which are consistent with the ‘estrogen augmented by progesterone' hypothesis [23–25] prompted suggestions that alternative ways of protecting the endometrium – that have no or reduced effects on breast tissue – should be tried, e.g. the use of an intra-uterine device with a progestin or the intravaginal administration of progesterone [23]. The elimination of progestins from HRT preparations was even suggested, since the increased risk of endometrial cancer with unopposed estrogens would be more than counterbalanced by the reduced risk of BC [18].