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Plan B

"Oct. 18, 2012 -- While the use of long-acting intrauterine devices (IUDs) is increasing, 1 in 9 women at risk for unintended pregnancies is not using any birth control, according to a new government report.

Researchers from the Natio"...

Plan B

Plan B

CLINICAL PHARMACOLOGY

Emergency contraceptives are not effective if the woman is already pregnant. Plan B® (levonorgestrel) is believed to act as an emergency contraceptive principally by preventing ovulation or fertilization (by altering tubal transport of sperm and/or ova). In addition, it may inhibit implantation (by altering the endometrium). It is not effective once the process of implantation has begun.

Pharmacokinetics

Absorption

No specific investigation of the absolute bioavailability of Plan B® (levonorgestrel) in humans has been conducted. However, literature indicates that levonorgestrel is rapidly and completely absorbed after oral administration (bioavailability about 100%) and is not subject to first pass metabolism. After a single dose of Plan B® (levonorgestrel) (0.75 mg) administered to 16 women under fasting conditions, maximum serum concentrations of levonorgestrel are 14.1 ± 7.7 ng/mL (mean ± SD) at an average of 1.6 ± 0.7 hours. No formal study of the effect of food on the absorption of levonorgestrel has been undertaken.

Table 1
Pharmacokinetic Parameter Values Following Single Dose Administration of Plan B (Levonorgestrel 0.75 mg) to Healthy Female Volunteers


  Mean (± S.D.)
N Cmax
(ng/mL)
Tmax
(h)
CL
(L/h)
Vd
(L)
T1/2
(h)
AUC
(ng/mL/h)
16 14.1 ± 7.7 1.6 ± 0.7 7.7 ± 2.7 260.0 24.4 ± 5.3 123.1 ± 50.1

Distribution

Levonorgestrel in serum is primarily protein bound. Approximately 50% is bound to albumin and 47.5% is bound to sex hormone binding globulin (SHBG).

Metabolism

Following a single oral dosage, levonorgestrel does not appear to be extensively metabolized by the liver. The primary metabolites are 3α,5β- and 3α,5β-tetrahydrolevonorgestrel with 16β-hydroxynorgestrel also identified. Together, these account for less than 10% of parent plasma levels. Urinary metabolites hydroxylated at the 2α and 16β positions have also been identified. Small amounts of the metabolites are present in plasma as sulfate and glucuronide conjugates.

Excretion

The elimination half-life of levonorgestrel following single dose administration as Plan B® (levonorgestrel) (0.75 mg) is 24.4 ± 5.3 hours. Excretion following single dose administration as emergency contraception is unknown, but based on chronic, low-dose contraceptive use, levonorgestrel and its metabolites are primarily excreted in the urine, with smaller amounts recovered in the feces.

Special Populations

Geriatric: This product is not intended for use in geriatric (age 65 years or older) populations and pharmacokinetic data are not available for this population.

Pediatric: This product is not intended for use in pediatric (premenarchal) populations, and pharmacokinetic data are not available for this population.

Race: No formal studies have evaluated the effect of race. However, clinical trials demonstrated a higher pregnancy rate in the Chinese population with both Plan B® (levonorgestrel) and the Yuzpe regimen (another form of emergency contraception consisting of two doses of ethinyl estradiol 0.1 mg + levonorgestrel 0.5 mg). The reason for this apparent increase in the pregnancy rate of emergency contraceptives in Chinese women is unknown.

Hepatic Insufficiency and Renal Insufficiency: No formal studies have evaluated the effect of hepatic insufficiency or renal insufficiency on the disposition of emergency contraceptive tablets.

Drug-Drug Interactions: No formal studies of drug-drug interactions were conducted.

CLINICAL STUDIES

A double-blind, controlled clinical trial in 1,955 evaluable women compared the efficacy and safety of Plan B® (one 0.75 mg tablet of levonorgestrel taken within 72 hours of intercourse, and one tablet taken 12 hours later) to the Yuzpe regimen (two tablets of 0.25 mg levonorgestrel and 0.05 mg ethinyl estradiol, taken within 72 hours of intercourse, and two tablets taken 12 hours later). Plan B® (levonorgestrel) was at least as effective as the Yuzpe regimen in preventing pregnancy. After a single act of intercourse, the expected pregnancy rate of 8% (with no contraception) was reduced to approximately 1% with Plan B® (levonorgestrel) .

Emergency contraceptives are not as effective as routine contraception since their failure rate, while low based on a single use, would accumulate over time with repeated use (see WARNINGS). See Table 2 below.

Table 2
Percentage of Women Experiencing an Unintended Pregnancy During the First Year of Typical Use and the First Year of Perfect Use of Contraception, and the Percentage Continuing Use at the End of the first Year- United States


  % of Women Experiencing an Unintended Pregnancy within the First Year of Use % of Women Continuing Use at One Year3
Method
(1)
Typical Use1
(2)
Perfect Use2
(3)
(4)
Chance4 85 85  
Spermicide5 26 6 40
Periodic Abstinence 25   63
  Calendar   9  
  Ovulation Method   3  
  Symptom-thermal6   2  
  Post-ovulation   1  
Withdrawal 19 4  
Cap7      
  Parous Women 40 26 42
  Nulliparous Women 20 9 56
Sponge      
  Parous Women 40 20 42
  Nulliparous Women 20 9 56
Diaphragm7 20 6 56
Condom8      
  Female (Reality) 21 5 56
  Male 14 3 61
Pill 5   71
  Progestin Only   0.5  
  Combined   0.1  
IUD      
  Progesterone T 2.0 1.5 81
  Copper T 380A 0.8 0.6 78
  LNg 20 0.1 0.1 81
Depo Provera 0.3 0.3 70
Norplant and Norplant-2 0.05 0.05 88
Female Sterilization 0.5 0.5 100
Male Sterilization 0.15 0.10 100

Emergency Contraceptive Pills: Treatment initiated within 72 hours after unprotected intercourse reduces the risk of pregnancy by at least 75%.9

Lactational Amenorrhea Method: LAM is a highly effective temporary method of contraception.10

Source: Trussell J. Contraceptive efficacy. In Hatcher RA, Trussell J, Stewart F, Cates W, Stewart GK, Kowal D, Guest F, Contraceptive Technology; Seventeenth Revised Edition. New York, NY: Irvington Publishers, 1998.

1.  Among typical couples who initiate use of a method (not necessarily for the first time), the percentage who experience an unintended pregnancy during the first year if they do not stop use for any other reason.
2.  Among couples who initiate use of a method (not necessarily for the first time) and who use it perfectly (both consistently and correctly), the percentage who experience an unintended pregnancy during the first year if they do not stop use for any other reason.
3.  Among couples attempting to avoid pregnancy, the percentage who continue to use a method for one year.
4.  The percentages of women becoming pregnant in columns (2) and (3) are based on data from populations where contraception is not used and from women who cease using contraception in order to become pregnant. Among such populations, about 89% become pregnant within one year. This estimate was lowered slightly (to 85%) to represent the percentage who would become pregnant within one year among women now relying on reversible methods of contraception if they abandoned contraception altogether.
5.  Foams, creams, gels, vaginal suppositories, and vaginal film.
6.  Cervical mucus (ovulation) method supplemented by calendar in the pre-ovulatory and basal body temperature in the post-ovulatory phase.
7.  With spermicidal cream or jelly.
8.  Without spermicides.
9.  The treatment schedule is one dose within 72 hours after unprotected intercourse and a second dose 12 hours after the first dose. The Food and Drug Administration has declared the following brands of oral contraceptives to be safe and effective for emergency contraception: Ovral (1 dose is 2 white pills), Alesse (1 dose is 5 pink pills), Nordette or Levlen (1 dose is 2 light-orange pills), Lo/Ovral (1 dose is 4 white pills), Triphasil or Tri-Levlen (1 dose is 4 yellow pills).
10.  However, to maintain effective protection against pregnancy, another method of contraception must be used as soon as menstruation resumes, the frequency or duration of breastfeeds is reduced, bottle feeds are introduced or the baby reaches six months of age.

Last reviewed on RxList: 5/24/2007
This monograph has been modified to include the generic and brand name in many instances.

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