Hormone Replacement Therapy and Breast Cancer: An Overview

Paolo Spriano

July 05, 2022

It is projected that by 2050, 1.6 billion women in the world will have reached menopause or the postmenopausal period, a significant increase, compared with a billion women in 2020. Of all menopausal women, around 75% are affected by troublesome menopause symptoms, such as hot flashes and night sweats. Around 84% of postmenopausal women experience genitourinary symptoms, such as vulvovaginal atrophy and incontinence.

Menopausal hormone therapy (MHT) is the most effective treatment for managing these symptoms; however, its effects on numerous aspects of female health remain uncertain, in particular with regard to breast cancer. The influence of MHT on breast cancer remains unsettled, with discordant findings from observational studies and randomized clinical trials, a factor that affects the decisions made by doctors concerning hormone therapy in menopausal women.


Conjugated equine estrogens (CEEs) were introduced into clinical practice in the 1940s. For decades, MHT was the main treatment in conventional medicine for the symptoms of menopause. MHT was used in Western countries for about 600 million women starting from 1970, and it progressively increased during the 1990s. Professional organizations recommended MHT for the prevention of osteoporosis and chronic heart disease (CHD), and a third of prescriptions were for women older than 60 years. Against this background, the National Institutes of Health launched randomized trials of MHT through the Women's Health Initiative (WHI) to test whether the association with reduced risk for CHD found in observational studies was real and to obtain reliable information on the overall risks and benefits regarding the prevention of chronic disease for postmenopausal women aged 50 to 79 years. The WHI trials tested standard-dose oral CEEs with and without standard-dose continuous medroxyprogesterone acetate (EPT). In 2002, the results of the WHI studies raised a series of concerns about the long-term safety of MHT, in particular the finding of an increased risk of breast cancer for women undergoing therapy. That risk exceeded the benefits from reductions in hip fractures and colorectal cancer.

The WHI findings received wide attention. Prescriptions for MHT dropped precipitously after 2002 and continued to decline in subsequent years. Declines were most marked for standard-dose EPT and in older women. The results of the CEE study were less negative, compared with those for EPT, as they showed no effect on CHD, a nonsignificant reduction in the risk of breast cancer, and a more favorable risk-benefit ratio for younger women, compared to older women. A decade later, it had become widely accepted that MHT should not be used for the prevention of chronic disease in older women; however, short-term use for treatment of vasomotor symptoms remains an accepted indication.

Risks and Outcomes

Emerging from a series of WHI reports are complex models on the effect of hormonal therapy on the risk and outcome of breast cancer. In one study, women with an intact uterus received CEEs plus medroxyprogesterone acetate (MPA). An increase in the risk of breast cancer was observed over a median of 5.6 years of treatment, followed by a moderate reduction, with the risk increasing after 13 years of cumulative follow-up. For women treated with CEE alone, the reduction in risk observed over an average of 7.2 years of treatment was maintained for 13 years of follow-up.

Results from observational studies contrast with those from randomized controlled trials, particularly those concerning the use of estrogens only. A meta-analysis by the Collaborative Group on Hormonal Factors in Breast Cancer showed that both EPT and CEE were associated with a higher risk of breast neoplasia. Results of the Million Women Study showed a higher death rate.

Treatment Methods and Duration

Information from prospective studies on the effects of commencing MHT at various ages between 40 and 59 years show that for women who commenced treatment at any time within this age range, the relative risk (RR) was similar and was highly significant for all ages. Few women had started MHT treatment well after menopause at ages 60 to 69 years, and their excess risks during years 5 to 14 of current use were significant for estrogen-progestogen, but not for estrogen-only MHT. If these associations are largely causal, then for women of average weight in developed countries, 5 years of MHT, starting at age 50 years, would increase breast cancer incidence at ages 50 to 69 years by about 1 in every 50 users of estrogen plus daily progestogen preparations; 1 in every 70 users of estrogen plus intermittent progestogen preparations; and 1 in every 200 users of estrogen-only preparations. The corresponding excesses from 10 years of MHT would be about twice as great.

During 5 to 14 years of MHT use, the RRs were similarly increased if MHT use had started at ages 40 to 44 years, 45 to 49 years, 50 to 54 years, and 55 to 59 years; RRs appeared to be attenuated if MHT use had started after age 60 years. They were also attenuated by adiposity, particularly for estrogen-only MHT (which had little effect in obese women). After MHT use ceased, some excess risk of breast cancer persisted for more than a decade; this is directly correlated with the duration of treatment.

Therefore, it can be expected that the effects of MHT may vary between participants on the basis of age or time since menopause, as well as treatments (MHT type, dose, formulation, duration of use, and route of administration). Regarding formulation effects on the risk of breast cancer, new evidence shows an increased risk of 28%. Progestogens appeared to be differentially associated with breast cancer (micronized progesterone: odds ratio [OR], 0.99; 95% CI 0.55 – 1.79; synthetic progestin: OR, 1.28; 95% CI, 1.22 – 1.35). When prescribing MHT, micronized progesterone may be the safer progestogen to use.

In conclusion, MHT has a complex balance of benefits and risk on various health outcomes. Some effects differ qualitatively between ET and EPT. Regarding use of MHT, consideration should be given to the full range of effects, along with patients' values and preferences. The overall quality of existing systematic reviews is moderate to poor. Clinicians should evaluate their scientific strength before considering applying their results in clinical practice. Regarding use of any hormone therapy regimen, consideration should be given to the full range of risk and benefits and should involve shared decision-making with the patient. It should be recognized that risk-benefit balance is altered by factors such as age, time from menopause, oophorectomy status, and prior hysterectomy and that some outcomes persist and that there is some attenuation after stopping use.

This article was translated from Univadis Italy.


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