Mechanism of Action
The mechanism of action of progestogen-only contraceptives depends on the progestogen activity and dose.
- Very-low-dose progestogen-only contraceptives, such as traditional progestogen-only pills (and subdermal implants Norplant and Jadelle and intrauterine systems Progestasert and Mirena), inconsistently inhibit ovulation in ~50% of cycles and rely mainly on their progestogenic effect of thickening the cervical mucus, thereby reducing sperm viability and penetration.
- Intermediate-dose progestogen-only contraceptives, such as the progestogen-only pill Cerazette (or the subdermal implant Nexplanon), allow some follicular development (part of the steps of ovulation) but much more consistently inhibit ovulation in 97–99% of cycles. The same cervical mucus changes occur as with very-low-dose progestogens.
- High-dose progestogen-only contraceptives, such as the injectables Depo-Provera and Noristerat, completely inhibit follicular development (see above) and ovulation. The same cervical mucus changes occur as with very-low-dose and intermediate-dose progestogens.
In anovulatory cycles using progestogen-only contraceptives, the endometrium is thin and atrophic. If the endometrium were also thin and atrophic during an ovulatory cycle, this could, in theory, interfere with implantation of a blastocyst (embryo).
Read more about this topic: Progestogen-only Pill
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