Hey PollyPerky,
I checked the evidence on transdermal versus oral estrogens. It looks like I overstated the benefits of transdermal estrogen in breast cancer risk (I couldn’t find much evidence for a benefit or a risk). Here is what I found in the International Menopause Society (IMS) 2016 guidelines:
“Currently available data imply no difference in risk [of breast cancer] between oral and transdermal routes of estradiol administration. However, there are not enough data from adequately powered clinical studies to fully evaluate possible differences in the incidence of breast cancer using different types, doses and routes of estrogen, type of progestogens and androgen administration.”
However, transdermals are associated with quite a few other benefits, including lower risk of venous thromboembolism (blood clots), stroke and weight gain. To avoid accusations of cherry-picking, I have stuck to reviews and international published guidelines here, and I’m happy to be corrected if I’m misrepresenting anything. 
[Review] Sood et al. Prescribing menopausal hormone therapy: an evidence-based approach. Int J Womens Health 2014;6 : 47–57. www.ncbi.nlm.nih.gov/pmc/articles/PMC3897322/
“[T]he type of MHT modulates risk, because estrogen alone appears to decrease the risk of breast cancer while combination regimens with estrogen and progestogen have been shown to increase this risk after 3–5 years of use. The mode of delivery of estrogen is also important because, in contrast with oral estrogen, low-dose transdermal estrogen appears to be linked to a lower risk of cholecystitis, stroke, and deep venous thromboembolism.”
Evidence seems fairly clear (but not entirely) that estrogen alone does not increase breast cancer risk, but that combinations with progestogens does.
[Review] Santen. Menopausal hormone therapy and breast cancer. J Steroid Biochem Mol Bio 2014; 142: 52–61. www.ncbi.nlm.nih.gov/pubmed/23871991
“These data suggest that estrogen alone neither decreases nor increases risk in younger women initiating therapy close to the time of menopause but decreases riskin older women. Both younger and older women experience an excess risk with estrogen plus a progestogen.”
Here are some excerpts from the NICE guidelines (NG23; www.nice.org.uk/guidance/ng23) concerning use of transdermal versus oral HRT:
Long-term benefits and risks of hormone replacement therapy.
Excerpts from 1.5.1:
- the risk of venous thromboembolism (VTE) is increased by oral HRT compared with baseline population risk
- the risk of VTE associated with HRT is greater for oral than transdermal preparations
- the risk associated with transdermal HRT given at standard therapeutic doses is no greater than baseline population risk.
1.5.2: Consider transdermal rather than oral HRT for menopausal women who are at increased risk of VTE, including those with a BMI over 30 kg/m2.
Excerpts from 1.5.6:
HRT with oestrogen alone is associated with no, or reduced, risk of coronary heart disease
HRT with oestrogen and progestogen is associated with little or no increase in the risk of coronary heart disease.
1.5.7: Explain to women that taking oral (but not transdermal) oestrogen is associated with a small increase in the risk of stroke. Also explain that the baseline population risk of stroke in women aged under 60 years is very low.
Excerpts from 1.5.11:
- the baseline risk of breast cancer for women around menopausal age varies from one woman to another according to the presence of underlying risk factors
- HRT with oestrogen alone is associated with little or no change in the risk of breast cancer
- HRT with oestrogen and progestogen can be associated with an increase in the risk of breast cancer
- any increase in the risk of breast cancer is related to treatment duration and reduces after stopping HRT.
Many of these recommendations are also backed up by the 2016 IMS Recommendations on women’s midlife health and menopause hormone therapy, www.imsociety.org/manage/images/pdf/4429e3dd302aac259ad68c3be7f60599.pdf
“Oral estrogen has been associated with a small but significant increase in fat mass and a decrease in lean mass, whereas lean body mass and fat mass are unaffected by transdermal estradiol.”
“The risk of ischemic stroke with MHT may be related solely to oral therapy, with lower doses having a smaller risk and no significant risk occurring with transdermal therapy.”
“Epidemiological studies have not found any increased risk of VTE with use of transdermal estrogen. There is also strong evidence that the type of progestin associated with estrogen is of importance.”
“The incidence of breast cancer varies in different countries. Therefore, currently available data may not be applicable everywhere. The degree of association between breast cancer and MHT remains controversial.”
The possible increased risk of breast cancer associated with MHT is small and estimated at less than 0.1% per annum, or an incidence of 51.0 per 1000 women per year of use. 51+4 It is similar or lower than the increased risks associated with common lifestyle factors such as reduced physical activity, obesity and alcohol consumption.
Thank you for correcting me regarding estrogen + progesterone for women with a uterus, and estrogen only for women without – I had muddled the two! Here is what the NICE guidelines say about that:
1.4.2: Offer women HRT for vasomotor symptoms after discussing with them the short-term (up to 5 years) and longer-term benefits and risks. Offer a choice of preparations as follows:
- oestrogen and progestogen to women with a uterus
- oestrogen alone to women without a uterus.
Thanks for prompting me to revise my understanding here – it’s been a little while since I studied this area. It’s quite an active area of research these days.