Breast cancer risk with combined estrogen and progestogen therapy may differ depending on the progestogen used. Progestins including chlormadinone acetate, cyproterone acetate, medrogestone, medroxyprogesterone acetate, nomegestrol acetate, norethisterone acetate, promegestone, and tibolone have all been associated with similarly increased risk of breast cancer. Some research has found that oral progesterone and dydrogesterone with short-term use (<5 years) may be associated with lower risk of breast cancer relative to other progestins. In the long-term however (>5 years), oral progesterone and dydrogesterone have been associated with significantly increased breast cancer risk similarly to other progestogens. The lower risk of breast cancer with oral progesterone than with other progestogens may be related to the very low progesterone levels and relatively weak progestogenic effects it produces.