Over a period of three years of continuous use, two women per 1000 users of the levonorgestrel rod implants would become pregnant because of a method failure (three year pregnancy rate of 0.2 per 100 women).

No data has been published on the current formulation of levonorgestrel rod implants beyond three years of use, thus the current recommendation is for three years of use.

However, there are some data based on blood levels of levonorgestrel, that suggest the duration of effectiveness for levonorgestrel rods may be longer than three years.

1. Sivin I, Lahteenmaki P, Ranta S, Darney P, Klaisle C, Wan L, et al. Levonorgestrel concentrations during use of Levonorgestrel Rod (LNg Rod) implants. Contraception 1997;55(2):81-5.
2. Sivin I, Viegas O, Campodonico I, Diaz S, Pavez M, Wan L, et al. Clinical performance of a new two-rod levonorgestrel contraceptive implant: a three-year randomized study with Norplant implants as controls. Contraception 1997;55(2):73-80.