Mechanism of action: why is ovulation not the main target?
Unlike combined oral contraceptives (COCs), pure progestogens work differently. Their contraceptive effect is achieved primarily through peripheral mechanisms, whereas central action (suppression of ovulation) is less pronounced. In the majority of women taking such drugs, a normal ovulatory cycle is preserved.
How exactly they prevent pregnancy:
- Changes in cervical mucus properties: there is a sharp increase in its viscosity, creating an insurmountable physical barrier to sperm progression.
- Effect on the fallopian tubes: their normal motility is impaired, making egg transport difficult.
- Endometrial transformation: the process of uterine mucosal preparation is disorganized, making implantation of the fertilized egg impossible.
Oral formulations ("mini-pills")
"Mini-pills" contain microdoses of hormones (from 30 to 500 mcg). Due to the lack of an estrogen component, their overall contraceptive efficacy is somewhat lower than that of COCs; however, they possess a key advantage — the complete absence of estrogen-dependent complications.
Typical representatives:
- Exluton (contains 500 mcg of lynestrenol).
- Cerazette (contains 75 mcg of desogestrel).
Who are "mini-pills" indicated for:
- Nursing mothers: these are the drugs of choice during lactation, as pure progestogens 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.
Rules of administration and side effects: The drugs require the strictest discipline. They are taken continuously, starting from the first day of the menstrual cycle, strictly at the same hours. The main drawback of the group is frequent menstrual irregularities and the development of acyclic uterine bleeding (metrorrhagia).
Progestogen-releasing intrauterine systems (IUS)
The most well-known representative of this group is the Mirena system. Structurally, it is a T-shaped polyethylene framework with flexible arms. The vertical stem contains a drug reservoir holding 52 mg of levonorgestrel.
Pharmacokinetics and action: A special membrane provides strictly metered hormone release — 20 mcg per day. The system is placed for up to 5 years.
The action of the IUS is predominantly local in nature:
- Potent suppression of endometrial proliferation (blocking implantation).
- Thickening of the cervical mucus.
- Direct inhibition of sperm motility and functional activity in the uterine cavity and tubes.
Systemic effect manifested as ovulation suppression is highly variable and observed in far from all patients.
Indications and safety: IUSs are ideal for long-term contraception (especially in parous women) and for endometrial protection against hyperplasia during estrogen replacement therapy. Because the drug's action is local, systemic adverse reactions occur rarely. In some cases, headaches, mood swings, and dermatological manifestations (acne, hirsutism) may be observed.
Briefly: answers to topical questions
What active substances are used in oral progestogen-only contraceptives ("mini-pills")?
Oral progestogen-only contraceptives ("mini-pills") use microdoses (30–500 mcg) of synthetic progestogens. The main active substances include:
- Levonorgestrel — 0.03 mg (Microlut).
- Desogestrel — 0.075 mg (Cerazette).
- Lynestrenol — 0.5 mg (Exluton).
- Ethynodiol — 0.5 mg (Demulen).
Norethisterone and other pure progestogens may also be used as active components.
What side effects are characteristic of oral progestogen-only contraceptives?
Oral progestogen-only contraceptives are characterized by side effects associated with changes in menstrual patterns. The main adverse reactions include:
- Menstrual irregularities — changes in bleeding regularity.
- Metrorrhagia — high probability of acyclic uterine spotting.
At the same time, this group of drugs is characterized by the absence of estrogen-dependent side effects and complications, and the systemic impact on the body is significantly lower compared to combined contraceptives.
What pharmacokinetic parameters are characteristic of the Mirena intrauterine system?
The Mirena intrauterine system is characterized by local metered release of the active substance. Main parameters:
- Release rate — a special membrane ensures the delivery of levonorgestrel at a dose of 20 mcg/day.
- Duration of action — the contraceptive effect of the system persists for 5 years.
- Total hormone content — the reservoir on the vertical stem contains 52 mg of levonorgestrel.
The drug's action is predominantly local, therefore systemic adverse reactions occur rarely.
To remember the mechanism of action of progestogens, use the "Three S" rule: Slime/Mucus (thickens, creating a barrier), Site/Milieu (endometrium not ready for implantation), Sperm (lose motility in the tubes). Meanwhile, ovulation itself often persists.
Class and exam questions
Why is the efficacy of progestin-only pills («minipills») lower than that of combined hormonal contraceptives?
This is due to the absence of the estrogen component. In combined preparations, it is the estrogens that provide reliable central suppression of ovulation, whereas pure progestogens work mainly peripherally.
Can progestin-only contraceptives be prescribed during breastfeeding?
Yes, these are first-line drugs. Pure progestogens are completely safe for the infant; they do not alter the volume and quality of breast milk, nor do they shorten the duration of lactation.
How do progestin-releasing intrauterine systems work?
They release microdoses of the hormone directly into the uterine cavity. This leads to endometrial thinning, thickening of the cervical mucus, and inhibition of sperm function, while the systemic impact on the body is minimal.
What else to study on this topic
- Emergency contraceptives: pharmacodynamics and principles of use
- Comparative profile of COCs and progestogen-only contraceptives
- Pharmacokinetics of levonorgestrel and desogestrel
- Metrorrhagia during "mini-pill" use: pathogenesis and management
- Effect of progestogens on dermatological status (acne, hirsutism)
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