The pharmacokinetics and pharmacodynamics of Implanon, a single-rod etonogestrel contraceptive implant.

This paper reviews pharmacokinetic and pharmacodynamic studies with Implanon, which provides serum etonogestrel levels sufficient to inhibit ovulation within 8 h of insertion. After a peak of 813 pg/ml at 4 days, levels reach steady state (200 pg/ml) after 4-6 months and remain sufficient to prevent ovulation for 3 years. Variability is lower than with Norplant. Etonogestrel levels are similar in most ethnic groups, but 40% higher in women weighing < 50 kg. After implant removal, etonogestrel is not detectable within 1 week. Implanon inhibits ovulation by preventing the mid-cycle luteinizing hormone peak. Although it initially suppresses follicular development and estradiol production, ovarian activity slowly increases after 6 months, and follicle stimulating hormone and estradiol levels are almost normal. Endogenous progesterone levels remain in the subovulatory range for > 3 years in most subjects. In ovarian ultrasound studies, ovulation occurred in < 5% of users after 30 months of use. Ovulation was observed in most women within 3-4 weeks of implant removal. The pharmacokinetics and pharmacodynamics of Implanon indicate that it has high contraceptive efficacy, as reflected in a zero pregnancy rate over 5,629 woman-years of use. Its excellent reliability, ease of use, and reversibility make Implanon a valuable addition to current contraceptives.