Emergency contraception

Physiological background and therapeutic window

The success of postcoital drugs directly depends on understanding the biological rhythms of the reproductive system. The time window during which medication is appropriate and effective ranges from 24 to 72 hours after coitus.

The biological rationale for these timelines is based on the vast difference in the lifespan of male and female gametes:

• Spermatozoa have 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, an unprotected act occurring a few days before ovulation can still lead to conception, necessitating prompt pharmacological intervention.

Sites of action and mechanisms

Emergency contraception drugs work at various stages of the reproductive process, creating multiple barriers to pregnancy. The main pharmacological targets include:

1. Effect on ovulation: suppression of oocyte release from the ovary or a significant delay in this process.
2. Blockade of fertilization: creating conditions that make the meeting and fusion of male and female gametes impossible.
3. Alteration of transport: disruption of the normal progression of the oocyte through the fallopian tubes into the uterine cavity.
4. Prevention of implantation: disruption of the nidation process, preventing an already fertilized ovum from attaching to the uterine wall.

Classification of agents used

In modern pharmacology, 5 basic groups of drugs 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 drugs

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 suppression of gonadotropin-releasing hormone secretion occurs, followed by a decrease in the release of pituitary gonadotropic hormones. Clinical outcome: reliable inhibition of ovulation. In addition, the drugs cause endometrial atrophy, which serves as an additional mechanism preventing implantation. Administration regimen: prescribed at a dosage of 400 mg. Administered twice with a mandatory 12-hour interval. The main condition is to take the drug within 72 hours after unprotected intercourse.

Mifepristone Pharmacologically, it is a synthetic progesterone antagonist. Two main regimens are used in gynecology:

1. Emergency postcoital contraception: prescribed once in a high dose—600 mg. The standard time window is within 72 hours after sexual intercourse.
2. Cyclic administration: the drug is taken at 200 mg daily from the 23rd to the 27th day of the menstrual cycle.

When prescribing mifepristone, the physician must warn the patient about a characteristic side effect—a potential delayed menstruation.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of using pure estrogens in emergency contraception
• Mechanism of action of combined estrogen-progestin preparations
• Pharmacodynamics of pure progestins in the postcoital period
• Effect of drugs on fallopian tube transport function
• Use of danazol: indications and limitations

Other topics in the section "Hormone preparations, their synthetic substitutes, and antagonists"

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