Levonorgestrel, Pharmacokinetics, Efficacy and Safety 43North Wales, PA) with generic forms including Take Action (Teva
Pharmaceuticals, North Wales, PA).
The failure rate of LNG is lower when it is used continuously as a regular
contraceptive than when it is used as a single dose for emergency
contraception (EC). When LNG-EC is used during the follicular phase, it can
inhibit or delay ovulation, and this interference depends on how much the
follicular phase is advanced [42]. The efficacy of LNG-EC decreases when the
interval between unprotected intercourse and treatment increases, and this
suggests that it does not interfere with implantation. Several studies have
looked at the actions of LNG-EC on endometrial histology and physiology.
Deleterious effects of LNG impairing endometrial receptivity to subsequent
implantation have not been found either in vivo [43-45], or in vitro [46, 47].
Defining the efficacy of LNG-EC as the ratio of observed/expected
pregnancies, the efficacy of LNG-EC when used before ovulation is 100% [48,
49]. When used after ovulation has occurred, the number of observed and
expected pregnancies is not statistically different, indicating that LNG-EC is
ineffective in interfering with any reproductive process subsequent to
ovulation. This finding is incompatible with the inhibition of implantation by
LNG-EC and is consistent with the mechanism of action of EC reported by
Gemzell-Danielsson [50]. LNG-EC is highly effective for preventing
unintended pregnancy when it is used prior to ovulation, but it is ineffective in
preventing pregnancy when used after ovulation. This is because it has no
effect on subsequent reproductive processes, including implantation of the
embryo [51].
Levonorgestrel Implant
LNG containing implants are a long-acting contraceptive implant for birth
control and was sold under the brand name Norplant. A continuous low dose
of LNG diffuses through the wall of each capsule. It prevents pregnancy by
multiple mechanisms including inhibition of ovulation, thickening of the
cervical mucus and thinning of the endometrium. It is 100% bioavailable with
plasma levels around 0.3 ng/ml over 5 years but levels are highly variable
depending on individual metabolism and body weight. Norplant is a highly
effective contraceptive method. The most common side effect experienced by
levonorgestrel implant users is menstrual problems, but serious bleeding
problems are not more frequent than in controls [52]. Norplant compared to
other implants took a longer time to remove [52, 53]. Norplant was