Combined hormone replacement therapy (HRT), which consists of estrogen plus progestin, is associated with increased risk of breast cancer. HRT promotes some breast cancer types more than others and certain women are more susceptible to the effects of HRT than others. Short-term use is safer than long-term use, but even short-term use can increase breast cancer risk. HRT increases risk more for perimenopausal and menopausal women than older women. Generally speaking, estrogen-only HRT is safer than combined HRT, 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.
This is a really interesting overview of the topic. I was never on hormone replacement drugs so I haven't paid much attention to this area of research but there is some other information in this overview that would have been worth knowing earlier ... such as breast tenderness (like the pms kind) is considered a risk factor for breast cancer. I also didn't learn, until after the fact, that high breast density is a risk factor for breast cancer.
Another thing I just learned is that women who get hot flashes are less likely to get breast cancer. In the same vein, if one gets hot flashes brought on my breast cancer treatments, one should be happy because it means you're less likely to get a recurrence. I guess I should learn to smile and express gratitude through my hot flashes.
If you're on hormone replacement therapy (HRT) or are considering it, you might find this overview worth reading.