Compared to no hormone use, the risk of breast cancer in women who use estrogen alone (ERT) is increased by one percent per year of use, whereas the risk in women using both estrogen and a progestin (HRT) is increased by 8 percent per year. This means that a woman on conventional HRT for five years has a 40 percent higher risk of breast cancer than a woman not using HRT. This is a huge increase.
The important thing to keep in mind is that progestins are not progesterone. Progesterone protects against breast cancer. Progestins are synthetic drugs that supposedly mimic real progesterone but in fact do not. There are many studies that show the difference between progesterone and the synthetic progestins, and, in particular, their differing effect on breast cells.
Since it is well established that excess estrogen can cause and promote breast cancer, why is the HRT combination worse than estrogen alone? Knowledgable doctors believe it’s because progestins occupy progesterone receptors more tenaciously than progesterone, and thereby block progesterone from its functions, one of which is essentially to reverse estrogen’s cancer-causing effects. Progestins convey different messages to target cells than real progesterone. Real progesterone activates a gene that slows breast cell proliferation and increases protective apoptosis (programmed cell death) whereas most progestins do not.
Unopposed estrogen is dangerous and estrogen combined with synthetic progestins is even more dangerous in terms of breast cancer risk. Remember, only real (natural) progesterone protects a women’s breasts. Cowan et al from Johns Hopkins reported 1981 that premenopausal women with progesterone deficiency are 5.4 times more likely to develop breast cancer than women with normal progesterone levels. The recent study is just another confirmation that synthetic progestins should not be used.