“In addition, endogenous progesterone may have different effects on exogenous progesterone or etonogestrel metabolism.”
“The reasons why we decided not … theirs were: (a) structural differences between progesterone … etonogestrel (i.e., on position 17, Fig. 1) may alter CYP specificity, (b) the endogenous nature of progesterone vs. … etonogestrel and (c) the differences in the study population, i.e., postmenopausal women vs. contraceptive implant users.”