Mechanism of Action: Why Isn't Ovulation the Main Target?
Unlike combined oral contraceptives (COCs), pure progestins work differently. Their contraceptive effect is achieved predominantly through peripheral mechanisms, whereas central action (suppression of ovulation) is less pronounced. In the majority of women taking these drugs, a normal ovulatory cycle is preserved.
How they prevent pregnancy:
- Alteration of cervical mucus properties: a sharp increase in viscosity creates an impassable physical barrier to sperm progression.
- Effect on the Fallopian tubes: normal motility is disrupted, hindering the transport of the oocyte.
- Transformation of the endometrium: the process of preparing the uterine mucosal lining is disorganized, making implantation of a fertilized ovum impossible.
Oral Forms ("Minipills")
"Minipills" contain microdoses of hormones (30 to 500 mcg). Due to the absence of an estrogen component, their overall contraceptive efficacy is somewhat lower than that of COCs, yet they possess a key advantage—the complete absence of estrogen-dependent complications.
Typical Representatives:
- Exluton (contains 500 mcg lynestrenol).
- Cerazette (contains 75 mcg desogestrel).
Indications for Minipills:
- Lactating mothers: these are the drugs of choice during lactation, as pure progestins do not affect the duration of breastfeeding or the volume and quality of milk.
- Patients in risk groups: smoking women and patients of late reproductive age for whom estrogen intake is undesirable.
Administration Rules and Side Effects: These drugs require strict discipline. They are taken continuously, starting from the first day of the menstrual cycle, strictly at the same hour. The main drawback of the group is frequent menstrual cycle disruption and the development of acyclic uterine bleeding (metrorrhagia).
Intrauterine Progestin-Releasing Systems (IUS)
The most well-known representative of this group is the Mirena system. Structurally, it is a T-shaped polyethylene frame with flexible arms. The vertical stem holds a reservoir containing 52 mg of levonorgestrel.
Pharmacokinetics and Action: A specialized membrane ensures strictly controlled, dose-dependent hormone release—20 mcg per day. The system is placed for up to 5 years.
The action of the IUS is predominantly local:
- Potent suppression of endometrial proliferation (implantation blockade).
- Thickening of cervical mucus.
- Direct inhibition of sperm motility and functional activity within the uterine cavity and fallopian tubes.
Systemic effects, such as the suppression of ovulation, are highly variable and observed in a minority of patients.
Indications and Safety: IUSs are ideal for long-term contraception (especially in parous women) and for protecting the endometrium against hyperplasia during estrogen replacement therapy. Because the drug acts locally, systemic adverse reactions are rare. In some cases, headaches, mood swings, and dermatological manifestations (acne, hirsutism) may occur.