LEVONORGESTREL CONCENTRATIONS DURING USE OF LEVONORGESTREL ROD (LNG ROD) IMPLANTS

In a three-year randomized trial that included 398 women, blood sample s were collected for the purpose of assaying levonorgestrel concentrat ions in women using a new two-rod contraceptive implant system or an e arlier implant formulation, Norplant(R)-2 implants. Sample collection was at 1, 3, 6, 9, and 22 months after placement and semiannually ther eafter through three years. Resulting assays and analyses showed that levonorgestrel concentrations of each implant formulation decreased si gnificantly with time after placement, with increasing body weight, an d with ponderal index. In the third year, several measures indicated t hat concentrations of the contraceptive drug were higher in women usin g the LNG ROD implants than in users of the original formulation. No p regnancies occurred among women in either group in the three years. Th is study provides evidence that the minimum levonorgestrel concentrati on needed to protect against pregnancy is below 200 pg/ml, and possibl y is below 175 pg/ml. (C) 1997 Elsevier Science, Inc.