[Serum levels of levonorgestrel during long-term use of Norplant]

Abstract

Objective: (1) Assessment of longest effective contraceptive duration of Norplant (containing levonorgestrel, LNG) by serum levonorgestrel levels. (2) To observe the possible relationships between the serum LNG levels of long-term use (5 years) and body weight.

Methods: To determine serum LNG levels in 230 women among 1,356 cases at various intervals between 1 and 11 years after implantation using RIA kits provided by WHO.

Results: The average serum levels of LNG in users of Norplant at the 1st, 3rd, 5th years were 1,273.5, 924.0 and 739.6 pmol/L respectively. From the 5th to 7th years average serum LNG levels showed almost a linear decline, and from the 7th through 10th year it did not show much changes. The average serum LNG level was (511.1 +/- 209) pmol/L. A significant negative correlation was found between serum LNG levels and body weight at the 5th years.

Conclusion: One set of Norplant implants may provide effective contraception for 8-10 years in women with body weights less than 60 kg.