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Levonorgestrel is a totally synthetic and biologically active progestin which exhibits no significant estrogenic activity and is highly progestational. The absolute configuration conforms to that of D-natural steroids. Levonorgestrel is not subjected to a “first-pass” effect and is virtually 100% bioavailable. Plasma concentrations average approximately 0.30 ng/mL over 5 years but are highly variable as a function of individual metabolism and body weight.
Diffusion of levonorgestrel through the wall of each capsule provides a continuous low dose of the progestin. Resulting blood levels are substantially below those generally observed among users of combination oral contraceptives containing the progestins norgestrel or levonorgestrel. Because of the range of variability in blood levels and variation in individual response, blood levels alone are not predictive of the risk of pregnancy in an individual woman.
At least two mechanisms are active in preventing pregnancy: ovulation inhibition and thickening of the cervical mucus. Other mechanisms may add to these contraceptive effects.
Levonorgestrel concentrations among women show considerable variation depending on individual clearance rates, body weight, and possibly other factors. Levonorgestrel concentrations reach a maximum, or near maximum, within 24 hours after placement with mean values of 1600 ± 1100 pg/mL. They decline rapidly over the first month partially due to a circulating protein, SHBG, that binds levonorgestrel and which is depressed by the presence of levonorgestrel. At 3 months, mean levels decline to values of around 400 pg/mL while concentrations normalized to a 60 kg body weight were 327 ± 119 (SD) pg/mL at 12 months with further decline by 1.4 pg/mL/month to reach 258 ± 95 (SD) pg/mL at 60 months. Concentrations decreased with increasing body weight by a mean of 3.3 pg/mL/kg. After capsule removal, mean concentrations drop to below 100 pg/mL by 96 hours and to below assay sensitivity (50 pg/mL) by 5 to 14 days. Fertility rates return to levels comparable to those seen in the general population of women using no method of contraception. Circulating concentrations can be used to forecast the risk of pregnancy only in a general statistical sense. Mean concentrations associated with pregnancy have been 210 ± 60 (SD) pg/mL. However, in clinical studies, 20 percent of women had one or more values below 200 pg/mL but an average annual gross pregnancy rate of less than 1.0 per 100 women through 5 years.
Although lipoprotein levels were altered in several clinical studies with the NORPLANT (levonorgestrel implants (unavailable in us)) SYSTEM, the long-term clinical effects of these changes have not been determined. A decrease in total cholesterol levels has been reported in all lipoprotein studies and reached statistical significance in several. Both increases and decreases in high-density lipoprotein (HDL) levels have been reported in clinical trials. No statistically significant increases have been reported in the ratio of total cholesterol to HDL-cholesterol. Low-density lipoprotein (LDL) levels decreased during NORPLANT (levonorgestrel implants (unavailable in us)) SYSTEM use. Triglyceride levels also decreased from pretreatment values.
Last reviewed on RxList: 1/5/2009
This monograph has been modified to include the generic and brand name in many instances.
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