Mechanism of action of hormonal preparations used for emergency contraception: a review of the literature

Effectiveness of the Yuzpe regimen of emergency contraception by cycle day of intercourse: implications for mechanism of action

OBJECTIVE

The purpose of this study was to provide evidence about the mechanism of action of the Yuzpe method of emergency contraception by examining effectiveness by cycle day of intercourse relative to ovulation.

METHODS

Through a literature search, we identified eight studies that present the number of women treated and outcome of treatment by cycle day of unprotected intercourse relative to expected day of ovulation. Using five sets of external estimates of conception probabilities by cycle day of intercourse among women not using contraception, we assessed and compared the effectiveness of the Yuzpe regimen by whether intercourse occurred on or before the second day before ovulation or afterward, and whether intercourse occurred on or before the first day before ovulation or afterward.

RESULTS

In 36 of the 45 pairs of estimates of effectiveness, based on eight separate studies and the eight studies combined and five different sets of conception probabilities by cycle day, effectiveness was higher-and in most cases substantially higher-when intercourse occurred on or before the second day before ovulation (day -2) than when it occurred later. When data were stratified by whether intercourse occurred on or before the day before ovulation (day -1), effectiveness was greater when intercourse occurred early in 43 of 45 pairs.

CONCLUSIONS

These results suggest that one hypothesized mechanism of action of the Yuzpe method, inhibiting implantation of a fertilized egg, is unlikely to be the primary mechanism of action.

The effects of levonorgestrel on various sperm functions

Two doses of 750-microg levonorgestrel at 12 h apart is one of the regimens for emergency contraception. The mechanism of action of this regimen is not fully known. We investigated whether levonorgestrel influences sperm functions and thereby, exerts contraceptive activity. The motility, acrosome reaction, zona binding capacity, and oocyte fusion capacity of human spermatozoa treated with 1, 10, and 100 ng/mL levonorgestrel for 3 h were evaluated. Levonorgestrel decreased the curvilinear velocity of the treated spermatozoa in a dose-dependent manner. A significant decrease in straight-line velocity, average path velocity and linearity were also found with 100 ng/mL levonorgestrel treatment. This concentration of levonorgestrel, but not others, also marginally decreased (p = 0.045) the zona binding capacity of the treated spermatozoa. The steroid had no effect on acrosome reaction but had a dose-dependent inhibition on spermatozoa-oocyte fusion. These data show that levonorgestrel affects sperm function only at high concentration and the contribution of these effects to emergency contraception is unlikely to be significant.

Contraceptive applications of estrogen

Estrogens are a primary component of several contraceptive methods: combined oral contraceptive pills, a combined injectable contraceptive, the combined contraceptive vaginal ring, the combination transdermal contraceptive patch, and combined emergency contraceptive pills. Contraceptive formulations that contain estrogen are referred to as combined contraceptives because they also contain some form of progestin. This article reviews the contraceptive methods containing estrogen, beginning with a discussion of combined oral contraceptive pills. Formulations and clinical management, mechanisms of action, noncontraceptive benefits of use, therapeutic uses in addition to contraception, side effects, contraindications to use, and drug-drug interactions are described. Information follows about the newer combined contraceptive products including the injection, vaginal ring, and patch. Finally, combined emergency contraceptive pills are reviewed. Thorough knowledge of the contraceptive methods containing estrogen enables clinicians to provide expert care for women using these products.

Progestin contraceptives

Progestin contraceptives offer unique advantages, allowing the clinician to tailor optimum regimens for selected women. This article reviews the various progestational drugs, regimens, and delivery systems available presently and in development, with reference to the clinical benefits and limitations of each method.

Emergency contraception

The last decade has seen a huge interest in emergency contraception (EC) because of the potential it has to reduce abortion rates. A variety of hormonal methods is available although mifepristone-arguably the best method-is only licensed in China. The intrauterine device is highly effective but its use is limited because of the technical skill required for successful insertion. The mechanism of action of both the Yuzpe regimen of EC and of levonorgestrel is poorly understood and for all methods there are serious methodological difficulties involved with calculating efficacy. Nevertheless the risks and side-effects of EC are negligible and the practicalities of prescribing it are extremely simple. Research and programmatic efforts should concentrate on improving availability if EC is to fulfil its promise as a public health intervention to reduce unwanted pregnancy.