Many studies have shown a link between increased breast cancer risk and hormone replacement therapy (HRT) to treat bothersome menopausal symptoms. While the results of all the studies aren't exactly the same, there is good evidence that:
- HRT increases the risk of being diagnosed with invasive breast cancer
- breast cancer risk increases within the first several years of HRT use; risk appears to increase the longer a woman uses HRT
- combination HRT (contains both estrogen and progesterone) increases breast cancer risk more than estrogen-only HRT
- high-dose HRT increases risk more than low-dose HRT
A study found that breast cancer risk was higher in women who used HRT during the first 5 years after menopause started compared to women who waited more than 5 years to start HRT.
This research is part of the very large Million Women Study. The Million Women Study included nearly 1.3 million women in the United Kingdom and was designed to identify links between diet and lifestyle factors and health problems such as cancer. The women, who were about 57 years old, started the study between 1996 and 2001.
More than half (55%) of the women had used some form of HRT at some time in their lives and 35% were using HRT when the researchers did the analysis. The researchers recorded 15,759 breast cancers diagnosed in the 1.3 million women.
More than 60% of the breast cancers were diagnosed in women who had used HRT at some point; 45% of the breast cancers were diagnosed in women using HRT when the cancer was diagnosed.
The researchers compared breast cancer risk in three groups of women:
- women who used HRT in the first 5 years after menopause started
- women who used HRT more than 5 years after menopause started
- women who never used HRT
They also considered whether the women used estrogen-only or combination HRT.
Compared to women who never used HRT, women who used estrogen-only HRT had:
- a 43% higher risk of breast cancer when HRT was used in the first 5 years after menopause started
- a 32% higher risk when HRT was used more than 5 years after menopause started
Compared to women who never used HRT, women who used combination HRT had:
- a 61% higher risk of breast cancer when HRT was used in the first 5 years after menopause started
- a 46% higher risk when HRT was used more than 5 years after menopause started
HRT increased the risk of hormone-receptor-positive cancer more than the risk of hormone-receptor-negative cancer.
Other research has shown that when a woman stops using HRT, her risk of being diagnosed with breast cancer stays higher for several years but later drops to the same level as a woman who never used HRT. In this study, breast cancer risk stayed higher during the first 2 years after HRT was stopped, but then went down to a risk level similar to a woman who never used HRT.
Menopausal side effects can dramatically reduce quality of life for some women. These women have to weigh the benefits of HRT against the risks. If you're having severe hot flashes or other menopausal side effects and are considering HRT, talk to your doctor about all of your options. Ask how you can minimize your breast cancer risk AND relieve your symptoms. Be sure to discuss the pros and cons of different types of HRT.
This study suggests that the breast cancer risk associated with HRT will be lower if you can wait to use HRT until 5 years after menopause starts. If you do decide to take HRT, ask if you can take an estrogen-only, low-dose formulation and try to take it for the shortest time possible. You also may want to ask about vaginal or transdermal HRT.
Learn more about more about menopause and ways to manage side effects on the Breastcancer.org Managing Menopausal Symptoms pages.