Plasma levels of levonorgestrel in women during longterm use of Norplant

Levonorgestrel plasma levels were determined by a specific radioimmunoassay in a group of 110 women at various intervals between 1 and 60 months fallowing implantation of six silastic capsules containing levonorgestrel (Norplant). The implants were placed subdermally in the volar aspect of the forearm. Blood samples were taken twice a week for six consecutive weeks in each subject. Blood drawn from the antecubital vein ipsilateral to the implant site contained two to three times more levonorgestrel than contralateral samples. Average systemic plasma levels represented by contralateral samples ranged from 0.35 ± 0.03 to 0.29 ± 0.02 ng/ml (Mean ± S.E.) in the first and fifth year, respectively. The difference between these values is statistically significant at the 5% level. The results of the regression analysis of individual levels suggest that the decline observed throughout the first five years is mainly due to an early decrease taking place in the first two years. When the sampling period was repeated in the same subjects two or three times 10 to 24 months apart, no significant difference was found. A significant negative correlation was found between plasma levonorgestrel levels and body weight and body surface. The data indicate that the contraceptive efficacy of Norplant during the first five years of use is associated with sustained release of levonorgestrel and is in keeping with the estimated life span of Norplant of seven years.