The mechanism of action of progestin only contraceptives depends on the progestin activity and dose. Intermediate dose progestin-only contraceptives like Nexplanon or Implanon allow some follicular development but inhibit ovulation in almost all cycles as the primary mechanism of action. Ovulation was not observed in studies of Implanon in the first two years of use and only rarely in the third year with no pregnancies. A secondary mechanism of action is the progestogenic increase in cervical mucus viscosity which inhibits sperm penetration. Hormonal contraceptives also have effects on the endometrium that theoretically could affect implantation, however no scientific evidence indicates that prevention of implantation actually results from their use. Pharmacodynamics<unk>  is a progestogen, or an agonist of the progesterone receptor. It is less androgenic than levonorgestrel and norethisterone, and it does not cause a decrease in sex hormone-binding globulin levels. However, it is still associated with acne in up to 13.5% of patients when used as an implant, though this side effect only accounts for 1.3% of premature removals of the implant. In addition to its progestogenic and weak androgenic activity,<unk>  binds to the glucocorticoid receptor with about 14% of the affinity of dexamethasone (relative to 1% for levonorgestrel) and has very weak glucocorticoid activity.<unk>  has no other hormonal activity (e.g., estrogenic, antimineralocorticoid). Some inhibition of 5α-reductase and hepatic cytochrome P450 enzymes has been observed with<unk>  in vitro, similarly to other 19-nortestosterone progestins. Relative affinities (%) of<unk>  and metabolites Compound<unk>  150 20 0 14 0 15 0 5α-Dihydroetonogestrel 9 17 0 ? ? ? ? Sources: Values are percentages (%). Reference ligands (100%) were prome-gestone for the , metribolone for the , for the , for the , aldosterone for the , for , and cortisol for . Sources: Pharmacokinetics The bioavailability of<unk>  when given as a subcutaneous implant or as a vaginal ring is 100%. Steady-state levels of<unk>  are achieved within one week upon insertion as an implant or vaginal ring. The mean volume of distribution of<unk>  is 201 L. The plasma protein binding of the medication is at least 98%, with 66% bound to albumin and 32% bound to sex hormone-binding globulin.<unk>  is metabolized in the liver by CYP3A4. The biological activity of its metabolites is unknown. The elimination half-life of<unk>  is about 25 to 29 hours. Following removal of an<unk> -containing implant, levels of the medication were below the limits of assay detection by one week. The major portion of<unk>  is eliminated in urine and a minor portion is eliminated in feces.
Etonogestrel
Levonorgestrel
Gestodene
Desogestrel
Norgestrel
Chloroethynylnorgestrel
5α-Dihydrolevonorgestrel
Hexestrol