In light of the above, using a COC containing E2 could lead to fewer negative effects than EE on the endometriosis tissue.37,38 Unlike some other progestins, NOMAC is a 19-norprogesterone derivative that binds specifically to the progesterone receptor, exerts strong antiestrogenic effects, and has potent antigonadotropic activity; it reaches a peak serum concentration within 4 hours after oral administration, with a half-life of ∼50 hours.39 Because of its long half-life compared with other contraceptive progestins it is able to cover the 4-day hormone-free interval. This evidence concerns the gene PGR and endometriosis.