One aspect of this study was to assess the relative risk of invasive breast cancer according to route of estrogen administration and the type of progestogen, versus absence of HRT. The study found that for any given route of estrogen administration (oral or transdermal), the relative risk varied significantly between the different progestogens. Estrogen + progesterone and estrogen + dydrogesterone were associated with “no”, “slight” or “non-significant” increases in risk. All other combinations of estrogen + progestogen were associated with an increased risk in invasive breast cancer, with most increases being statistically significant.
