Combined hormone replacement therapy (HRT), which consists of estrogen plus progestin, is associated with increased breast cancer risk, although studies are not in agreement as to its extent. HRT is also known as menopausal hormone therapy. Certain women are more susceptible to its effects.
Short-term use is safer than long-term use, but even short-term use can increase risk. HRT increases risk more for perimenopausal and menopausal women than older women. One study reported that HRT use cancels out the benefits of exercise in reducing breast cancer risk. Generally speaking, estrogen-only hormone therapy is safer than combined HRT with respect to breast cancer, but this does not hold for high-risk women who might already have small, undetected tumors. Any form of HRT appears to be unsafe for breast cancer survivors.

Combined HRT is associated with increased breast cancer risk

Combined HRT is a well established risk factor for breast cancer. Strong evidence that combined HRT use could increase the risk of breast cancer was first published in 2002, when it was reported that participants in the Women’s Health Initiative trial who were taking combined HRT had higher rates of breast cancer than those who were not. Subsequent studies have confirmed the finding that combined HRT promotes breast cancer:
  • A 2024 Norwegian study, which included 1,275,783 women over 45 years that were followed for a median of 12.7 years, reported that combined HRT was associated with more than double the risk of breast cancer compared to non-use. The risk was highest for luminal A breast cancer. Vaginal estradiol was not found to be associated with breast cancer risk.
  • A Canadian study reported that percent mammographic density was 6% greater in postmenopausal breast cancer patients who used HRT than in never users of HRT with breast cancer. Cancer-free controls who used HRT had 1.6% greater breast density than never users of HRT. In other words, differences in percent mammographic density associated with HRT were greater in women who later developed breast cancer than in cancer-free controls.
  • Another Canadian study that was designed to determine whether the decline in use of HRT since 2002 was followed by a concurrent decrease in breast cancer incidence reported that the largest decline in HRT use (between 2002 and 2004) took place concurrently with a 9.6% reduction in the incidence of breast cancer.
Breast cancer rates declined in every country for which data has been analyzed after the publication of the Women’s Health Initiative results caused a drop in the use of combined HRT. Note that use of birth control pills or other hormonal birth control increases the breast cancer risk associated with HRT among perimenopausal women.
  • A 2024 review reported that combined HRT for women over 50 resulted in a significantly increased risk of breast cancer (compared to no use) if taken for up to five years. Bioidentical hormones were also linked to increased risk.
  • A 2023 study concluded that the significant reduction in use of HRT since 2003 provides a credible explanation for most of the reduction in breast cancer among U.S. postmenopausal women during the past two decades.
  • A 2022 study found a clear association between HRT use and increased breast density, a strong risk factor for breast cancer.
  • A 2021 meta-analysis of data from four previous randomized controlled trials reported that the risk of relapse was significantly higher in women using HRT who had hormone receptor-positive (ER+/PR+) breast cancer, but not in those with hormone receptor-negative tumors.
  • A study reported that use of HRT was associated with increased risk of progression of DCIS or LCIS to invasive breast cancer.
  • A study reported that coffee consumption could increase breast cancer risk among postmenopausal women who previously used HRT.
  • A follow-up report describing Women's Health Initiative study outcomes after an average of 11 years found that combined HRT was associated with more invasive breast cancers (compared with placebo) among HRT users (a rate of 0.42% per year) compared to nonusers (0.34% per year). Breast cancers in the combined HRT group were more likely to be lymph node-positive. There were also more breast cancer-related deaths in the combined HRT group.

HRT promotes some breast cancer types more than others

Several studies have reported that while HRT use is associated with heightened risk of all breast cancer subtypes, it is most strongly associated with lobular breast cancer. A Chinese study reported that the risk of ER+/PR- breast cancer, a relatively rare subtype that is more common in Asia, was more than doubled by HRT use.

Increased breast cancer risk from HRT is biologically plausible

Increased risk of breast cancer as a result of HRT use is plausible. Women with menopausal symptoms have approximately half the breast cancer risk of those who do not experience them. Those with symptoms have lower estrogen levels as a result of going through menopause compared to women without symptoms. One study designed to examine whether those with menopausal symptoms have reduced breast cancer risk reported that the greater the intensity of hot flashes, the lower the risk of breast cancer. Women with the lowest levels of estrogen (who are likely to have the most severe menopausal symptoms) when starting combined HRT have been reported to experience the greatest increase in breast cancer risk from HRT. Similarly, underweight (BMI<20 kg/m2) women (who are also likely to have lower circulating estrogen) have a higher risk of breast cancer associated with HRT use than obese (BMI>30 kg/m2) women.
New onset of hot flashes as a result of anti-estrogen treatments such as aromatase inhibitors and tamoxifen have also been found to be associated with favorable breast cancer prognosis. Since HRT reduces menopausal symptoms, it might also extinguish the risk advantage conferred by having a sudden and dramatic drop in estrogen and other hormones as a result of menopause. However, this does not explain why estrogen-only hormone therapy increases breast cancer risk less than combined HRT. Also, one large prospective study found no association between hot flashes and breast cancer risk.

Certain women are more susceptible

Some women appear to be more susceptible to the cancer-promoting effects of combined HRT than others. For example, women with tender or dense breasts have been shown to be more vulnerable to the risk-increasing effects of combined HRT. Alcohol consumption also appears to heighten the risk of breast cancer associated with HRT. There is also some evidence that women with a family history of breast cancer are more susceptible to the breast cancer-promoting effects of HRT.

Increased breast tenderness from combined HRT linked to heightened risk

One trial reported that breast tenderness during combined HRT use was associated with increased subsequent breast cancer risk, especially among women who experienced breast tenderness before beginning HRT. To conduct the study, the authors analyzed data from the Women’s Health Initiative Estrogen plus Progestin clinical trials. The trials included 16,608 cancer-free postmenopausal women who had not had hysterectomies and were randomly assigned to receive either daily estrogen plus progestin or a placebo, as well as 10,739 women on estrogen-only or placebo. Self-reported breast tenderness was recorded at baseline and one year later.
Use of combined HRT was found to double the risk of invasive breast cancer among women with baseline breast tenderness. On the other hand, combined HRT had smaller and nonsignificant effects in women without baseline breast tenderness. The risk of new-onset breast tenderness was significantly higher among women assigned to HRT compared to placebo. New-onset breast tenderness was also associated with higher breast cancer risk among trial participants assigned to combined HRT, but not among women assigned to estrogen alone. New-onset breast tenderness during use of combined HRT was also found to be associated with increased risk of subsequent breast cancer.

Combined HRT increases the already high risk associated with dense breasts

Combined HRT increases breast cancer risk more for women with dense breasts than those with low breast density. One study that examined the associations between breast density, HRT and risk of breast cancer examined data concerning 587,369 women who underwent 1,349,027 screening mammograms. Of these women, 14,090 were eventually diagnosed with breast cancer.
Risk of breast cancer was found to be low for women with low breast density. Among women aged 55 to 59 years with low breast density, the five-year risk was 0.8% for non-HRT users and 0.9% for combined HRT users. On the other hand, breast cancer risk was relatively high among women with very high density, particularly for combined HRT users. Among women age 55 to 59 years with high breast density, the five-year risk was 2.4% for non-HRT users, 3.0% for estrogen-only hormone therapy users, and 4.2% for combined HRT users. In addition, risk of advanced-stage breast cancer was increased 1.7-fold for postmenopausal HRT users with very high density compared to those with average breast density. In other words, combined HRT can amplify the already heightened risk of breast cancer associated with high breast density.

Alcohol consumption increases risk

Alcohol increases the breast cancer risk associated with HRT. A study of 40,680 postmenopausal women in the California Teachers Study reported increased ER+ breast cancer risk associated with alcohol intake among women who were current HRT users. Risks were similar for combined and estrogen-only hormone therapy.

HRT after oophorectomy can influence survival of BRCA carriers

A 2021 study reported that use of HRT to counteract menopausal symptoms after prophylactic removal of the ovaries appeared to be safe up to age 45 but tripled breast cancer risk among BRCA1 and BRCA2 mutation carriers over 45. The risk conferred by HRT use appears to be related to time of exposure. i.e., it is greater starting around the natural age of menopause.

Timing of HRT influences breast cancer risk

The timing of HRT, including the age at which it is started and the duration of use, determines the degree of risk. HRT use is most harmful before menopause; the excess risk of breast cancer declines with age and appears to disappear by age 65. Although HRT use appears to increase breast cancer risk almost immediately, long-term use (over five years) heightens the risk. When initiated close to menopause, even short durations of combined HRT use are associated with increased breast cancer risk.
A French prospective study reported that combined HRT use influences breast cancer risk in relation to the time that has elapsed between the onset of menopause and the initiation of HRT. The study included 53,310 postmenopausal women who were followed for an average of 8.1 years, during which time 1,726 developed breast cancer. The risk of breast cancer increased 1.5-fold for short-term (two years or less) use started during the first three years following the onset of menopause, whereas there was no increase in breast cancer risk for short-term use initiated after the three-year mark. In other words, women who started HRT at some point during the three years after menopause had an elevated risk of breast cancer even if the HRT was taken for only two years or less. Note that the combined HRT regimens typically used in France are formulated differently from U.S. regimens.

Safety of estrogen-only hormone therapy

Studies that have compared combined HRT and estrogen-only hormone therapy have consistently reported that estrogen-only therapy is associated with significantly lower additional breast cancer risk. For example, an update of results of the Women's Health Initiative (WHI) trial reported that use of conjugated equine estrogens alone (estrogen-only hormone therapy) was associated with lower risk of breast cancer. However, note that in the U.S. combined HRT is typically used in women who have not had a hysterectomy since estrogen-only hormone therapy increases the risk of endometrial cancer and progestin minimizes this risk. In addition, estrogen-only hormone therapy has been reported to be associated with increased ovarian cancer incidence and mortality.

Caution is advised

Unlike the case of many other postmenopausal breast cancer risk factors, the increased risk associated with combined HRT becomes detectable in less than two years of use and the excess risk also disappears quickly after cessation of short-term HRT use. This pattern suggests that combined HRT might act in the short term by stimulating the growth of existing micro-tumors, in addition to promoting the generation of new tumors with longer-term use.
Similarly, estrogen-only hormone therapy appears to be protective against breast cancer only for lower-risk women. The interaction between breast cancer development and estrogen is not straightforward. For example, childhood exposure to estrogen or estrogenic compounds found in soybeans and cow's milk are known to reduce breast cancer risk in adulthood. Even in adulthood, consumption of some phytoestrogens are associated with reduced breast cancer risk whereas others increase it under certain circumstances. Therefore, it is not farfetched to suggest that estrogen-only hormone therapy could protect against breast cancer for many women. The problem is that for women with existing but undetected tumors (some of which might not otherwise have developed into life-threatening disease), estrogen-only hormone therapy could stimulate tumor growth. Estrogen-only therapy also has been linked to increased ovarian cancer risk. Nor are bioidentical hormones necessarily safe in this circumstance. It appears unwise for breast cancer survivors to use any form of HRT.
Below are links to 20 recent studies concerning this topic. For a more complete list of studies, please click on HRT.