Plan B: A Progestin-Only Emergency Contraceptive

The FDA has approved marketing of Plan B (Women’s Capital Corporation), an emergency contraceptive "pill pack" that contains two 0.75-mg tablets of levonorgestrel. It is the second product to be approved for this indication. The Preven Emergency Contraceptive Kit (MedicalLetter, 40:102, 1998) includes four tablets, each containing 50 µg of ethinyl estradiol and 0.25mg of levonorgestrel, and a pregnancy test to rule out a pre-existing pregnancy, which would be a contraindication to taking the hormones.

MECHANISM OF ACTION — The mechanism by which high doses of estrogens or progestins taken after coitus prevent pregnancy is unclear. They can inhibit or delay ovulation.

Some studies have shown alterations in the endometrium, suggesting that they could also interfere with implantation of a fertilized egg, but other studies have found no such effects. Other possible mechanisms include interference with fertilization or with tubal transport of the embryo (A Glasier, N Engl J Med, 337:1058, 1997).

PROBABILITY OF PREGNANCY — Unprotected intercourse three days before ovulation results in pregnancy in about 15% of women, one or two days before ovulation in about 30%, and on the day of ovulation in about 12% (J Trussell et al, Contraception, 57:363, 1998). More than two days after ovulation, the probability of pregnancy approaches zero.

EFFECTIVENESS — A randomized controlled trial compared levonorgestrel 0.75 mg alone with ethinyl estradiol 100 µg and levonorgestrel 0.5 mg together. Both were taken twice 12 hours apart, within 72 hours after coitus. Pregnancy occurred in 11 of 976 women (1.1%) with levonorgestrel alone and in 31 of 979 (3.2%) with ethinyl estradiol plus levonorgestrel (Task Force, Lancet, 352:428, 1998). The sooner the drug is taken after coitus, the more effective the treatment; delaying the first dose by 12 hours increased the risk of pregnancy by almost 50% (G Piaggio et al, Lancet, 353:721, 1999).

ADVERSE EFFECTS — Nausea and vomiting can occur, but less frequently than with the estrogen-progestin combination. In the controlled trial, nausea occurred in 50% of women and vomiting in 19% taking the estrogen-progestin combination, compared to a 23% incidence of nausea and a 6% incidence of vomiting with levonorgestrel alone. Breast tenderness can occur with either regimen. No fetal malformations caused by unsuccessful use of high-dose oral contraceptives for emergency contraception have been reported.

DOSAGE AND COST — One 0.75-mg levonorgestrel tablet should be taken as soon as possible after coitus, but at least within 72 hours, and the other 12 hours later. Patients who vomit within one hour of taking either dose can repeat it, but presumably that would require purchase of another package or finding another source of the tablets. One Plan B pill pack costs about $21, compared to about $20 for the Preven kit. Plan B is currently available only from Women’s Capital Corporation (800-330-1271).

CONCLUSION — Use of levonorgestrel alone in the Plan B emergency contraceptive product as soon as possible after unprotected coitus can decrease the risk of pregnancy. It ap-pears to be more effective and better tolerated than estrogen-progestin combinations.

Note: Plan B has also been approved in Canada.

Reproduced from: The Medical Letter, Vol 23, No. 22, February 11, 2021