What is the Connection Between Hormone Replacement Therapy and Breast Cancer?

For several decades hormone replacement therapy (HRT) was thought the gold standard treatment for menopausal women to reduce symptoms like hot flashes, mood distortions, vaginal dryness and others. A combination of estrogen and progestin was used frequently and thought safe for women. As of 2002, safety of this treatment has been repeatedly called into question because statistically significant studies have shown that HRT has many risk factors. Among them, hormone replacement therapy and breast cancer risk often go hand in hand, and many physicians now view HRT as a treatment of last resort, instead of first preference.

Subsequent studies have verified that hormone replacement therapy and breast cancer are linked. In particular, there is evidence that HRT may augment tumors but make them less visible on mammogram results. This means women are more likely to be diagnosed with breast cancer in later stages of the disease where likelihood of total remission or cure is not as high. The estrogens in HRT are also thought to directly contribute to the growth of cancer in about 66% of tumors, since these cancers contain estrogen receptors. Giving estrogen feeds the tumor and can increase its size and strength.

Additional research in 2010 suggests that the connection between hormone replacement therapy and breast cancer strengthens with length of use. Long term users of HRT, who are defined as using HRT for five years or more, are more likely to get breast cancer, and to develop more aggressive types of the condition that increase mortality. This would suggest that if HRT is to be used at all, the planned duration of use should be very short. In the past, many women stayed on these treatments for ten years or more, greatly increasing their risk of early mortality due to breast cancer or other cancers.

The evidence compiled in a number of studies point to clear correlations between hormone replacement therapy and breast cancer, but HRT is still available in most areas. In the U.S., for example, hormone therapy may be used after total hysterectomy or could be recommended if osteoporosis is severe. Its continued availability leaves it open to off-label, more traditional use for menopausal symptoms, and doctors may still prescribe or recommend it to patients. Perhaps its use is warranted, but its potential for life-threatening adverse effects needs to be fully contemplated by patients before it is used.

There is also some question about the safety of hormonal birth control, which uses the same hormones. Thus far, there is a perceptible slight increase in risk of breast cancer that lasts for approximately 10 years after the last use, but then tends to normalize with women who have not used hormonal birth control. It’s still not known whether hormonal birth control and then HRT might have a cumulative effect that heightens breast cancer risk. Possibly, augmenting normal hormones during the years when women are producing them is less dangerous than adding them after production of these hormones has declined.