The "mini pill" is a progestogen-only oral contraceptive.

Progestogen Only Pills (POPs) are contraceptive pills that only contain synthetic progestogens (progestins) and do not contain oestrogen. Although such pills are sometimes called "Progesterone Only Pills," they do not actually contain progesterone, but one of several chemically related compounds.

Mechanism of action of the mini pill

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 and thereby reducing sperm viability and penetration.

Intermediate dose progestogen-only contraceptives, such as the progestogen-only pill Cerazette (or the subdermal implant Implanon), allow some follicular development 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 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 was also thin and atrophic during an ovulatory cycle, this could theoretically interfere with implantation of a blastocyst (embryo).

Efficacy / effectiveness of the mini pill

The theoretical efficacy is similar to that of the combined oral contraceptive pill (COCP). However, they are taken continuously without any breaks between packets and traditional progestogen-only pills must be taken to a much stricter time every day (within 3 hours vs. a COCP's 12 hours, although in some countries the POP Cerazette has an approved window of 12 hours). The real-life efficacy is therefore dependent upon user compliance.

POPs are not dependent upon gut bacterial flora for their absorption and so are not affected by courses of antibiotics. They will, however, be affected by any episodes of diarrhoea or vomiting.

Benefits of the mini pill

Lacking the oestrogen of combined pills, they are not associated with increased risks of DVT or heart disease. With the decreased clotting risk, they are not contraindicated in the setting of sickle-cell disease. The low dose of progestogen, and absence of oestrogen, make the minipill safe to use during breastfeeding; in fact, it may increase the flow of milk. Like combined pills, the minipill decreases the likelihood of pelvic inflammatory disease.

It is unclear whether POPs provide protection against endometrial cancer and ovarian cancer to the extent that COCP do.

Side effects of the mini pill

  • With no break in the dosage, bleeds do not initially occur at a predictable time. Most women tend to establish, over a few months, light spotting at approximately regular intervals.
  • May cause mastalgia (breast pain) and mood swings.
  • Weight gain is less commonly experienced than on the combined OCP.
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