Progestogen-only pill

Although such pills are sometimes called "Progesterone-Only Pills", they do not actually contain progesterone, but one of several chemically related compounds; and there are a number of progestogen-only contraceptive formulations.

Mechanism of action

The mechanism of action of progestogen-only contraceptives depends on the progestogen activity and dose.^[2]

• 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).

Efficacy and effectiveness

The theoretical efficacy is similar to that of the combined oral contraceptive pill (COCP). However, this pill is 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). However, in some countries, the POP Cerazette has an approved window of 12 hours. The effectiveness is, therefore, dependent upon compliance.

Benefits

Lacking the estrogen 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 progestin-only pill is recommended over regular birth control pills for women who are breastfeeding because the mini-pill does not affect milk production (estrogen reduces the amount of breast milk). Like combined pills, the minipill decreases the likelihood of pelvic inflammatory disease.

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

There are fewer serious complications than on COCP.^[3]

Side-effects

• With no break in the dosage, menstrual flow does 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 tenderness) and mood swings.
• Some women may experience abdominal cramps and heavy bleeding.

Breast cancer risk

Epidemiological evidence on POPs and breast cancer risk is based on much smaller populations of users and so is less conclusive than that for COCPs.

In the largest (1996) reanalysis of previous studies of hormonal contraceptives and breast cancer risk, less than 1% were POP users. Current or recent POP users had a slightly increased relative risk (RR 1.17) of breast cancer diagnosis that just missed being statistically significant. The relative risk was similar to that found for current or recent COCP users (RR 1.16), and, as with COCPs, the increased relative risk decreased over time after stopping, vanished after 10 years, and was consistent with being due to earlier diagnosis or promoting the growth of a preexisting cancer.^[4][5]

The most recent (1999) IARC evaluation of progestogen-only hormonal contraceptives reviewed the 1996 reanalysis as well as 4 case-control studies of POP users included in the reanalysis. They concluded that: "Overall, there was no evidence of an increased risk of breast cancer".^[6]

Recent anxieties about the contribution of progestogens to the increased risk of breast cancer associated with HRT in postmenopausal women such as found in the WHI trials^[7] have not spread to progestogen-only contraceptive use in premenopausal women.^[2]

See also

• Oral contraceptive formulations

References