Physiological Rationale and Therapeutic Window
The success of postcoital medications directly depends on understanding the biological rhythms of the reproductive system. The time window during which drug administration is appropriate and effective ranges from 24 to 72 hours after coitus.
The biological rationale for these timeframes is based on the massive difference in the lifespan of male and female gametes:
- Spermatozoa exhibit high survival rates. Upon entering the female reproductive tract, they can maintain their fertilizing capacity for a prolonged period—from 3 to 7 days.
- The unfertilized oocyte, by contrast, is extremely vulnerable. Following ovulation, its period of viability is limited to just 12–24 hours.
Thus, unprotected intercourse occurring several days before ovulation can still lead to conception, which dictates the need for rapid pharmacological intervention.
Targets and Mechanisms of Action
Emergency contraception drugs work at various stages of the reproductive process, creating multiple barriers to pregnancy. The main pharmacological targets include:
- Effect on ovulation: suppression of oocyte release from the ovary or significant delay of this process.
- Blockade of fertilization: creating conditions that make the meeting and fusion of male and female gametes impossible.
- Alteration of transport: disruption of the normal progression of the oocyte through the uterine tubes into the uterine cavity.
- Prevention of implantation: disruption of the nidation process, preventing a fertilized ovum from attaching to the uterine wall.
Classification of Agents
In modern pharmacology, 5 basic groups of medications are distinguished for emergency hormonal contraception. The choice of a specific agent depends on the clinical situation:
- Estrogens.
- Combined estrogen-progestin preparations.
- Progestins.
- Danazol.
- Mifepristone.
Pharmacological Profile of Specific Agents
Partial Progestin Receptor Agonists The mechanism of action of this group is based on central effects. The drugs stimulate progestin receptors located in the hypothalamic-pituitary system. This triggers a cascade of reactions: a sharp inhibition of gonadotropin-releasing hormone secretion occurs, followed by a decrease in the release of pituitary gonadotropic hormones. Clinical result: reliable inhibition of ovulation. In addition, the drugs cause endometrial atrophy, serving as an additional mechanism preventing implantation. Regimen: administered at a dose of 400 mg. Taken twice with a mandatory 12-hour interval. The main condition is to administer the drug within 72 hours after unprotected intercourse.
Mifepristone Pharmacologically, this is a synthetic progesterone antagonist. Two main regimens are used in gynecology:
- Emergency postcoital contraception: administered as a single high dose of 600 mg. The standard time window is within 72 hours after sexual intercourse.
- Cyclical administration: the drug is taken at 200 mg per day between days 23 and 27 of the menstrual cycle.
When prescribing mifepristone, the clinician must warn the patient about a characteristic side effect—possible menstrual delay.