There is convincing evidence that using HRT raises a woman’s risk of cancer. There are other health concerns as well.
According to the International Agency for Research on Cancer, combined HRT for menopause is a known cause of breast cancer, mainly in women who recently used or are still using the therapy.
In 2002, a large clinical trial in the United States called the Women’s Health Initiative (WHI), which involved 16,600 women aged 50-79, reported a 25% increase in breast cancer risk among women who took combined HRT compared to those who took a placebo. This meant roughly 8 extra cases of breast cancer occurred for every 10,000 women taking combined HRT. Women on combined HRT were also more likely than those taking the placebo to have breast cancer found at a more advanced stage and to die from breast cancer. They were also more likely to have abnormal mammograms.
A number of follow-up studies from the WHI have since been published. The follow-up studies, along with other international research on combined HRT, have provided a better picture of the benefits and risks offered by combined HRT use in menopause. The risk of breast cancer is greatest when combined HRT is used:
- for 5 years or more (the risk increases the longer a woman uses HRT)
- by older postmenopausal women, especially those over the age of 60
- by women who start taking HRT closer to menopause
- by slim women
- by women with dense breasts
The WHI also included a clinical trial comparing the use of estrogen-only HRT to a placebo. Over 10,000 postmenopausal women who no longer had a uterus took part. Women who took estrogen and had no family history of breast cancer and no history of benign (non-cancerous) breast disease had a slightly lower risk of breast cancer compared to women who took the placebo.
Overall, studies suggest that both combined and estrogen-only HRT increase a woman’s risk of ovarian cancer but that the risk is low. Research done by the Collaborative Group on Epidemiological Studies of Ovarian Cancer showed that women who took HRT have a higher risk of ovarian cancer. How recently they used HRT was the most important factor in that risk. Women currently using HRT had the greatest increased risk compared to women who used it in the past. The risk went down over time once a woman stopped taking HRT. The Collaborative Group study also showed that the risk increases with the length of time a woman has used HRT.
The research is mixed on the effect of combined HRT on colorectal cancer. Some studies suggest a decreased risk (a protective effect) among users of combined HRT, while others do not. The WHI study found no effect of estrogen-only therapy on colorectal cancer, but some studies suggest a decreased risk (a protective effect) of colorectal cancer in women taking estrogen-only therapy.
Other health concerns
According to the WHI, regular use of combined HRT appears to increase the risk of heart disease, stroke and pulmonary embolism (blood clots in the lungs). Women taking combined HRT were more likely to require endometrial biopsies and need follow-up testing for vaginal bleeding. Using combined HRT regularly seemed to decrease the risk of hip fractures.
The WHI found that women taking estrogen-only HRT had an increased risk of stroke and venous thrombosis (a blood clot in a vein), but they had a decreased risk for bone fractures.