Oral contraceptives come in a variety of formulations. The main division is between combined oral contraceptive pills, containing both estrogen and synthetic progestogens (strictly progestins, the term used in the United States, but the former term is used in the United Kingdom), and progestogen only pills (mini-pills). Combined oral contraceptive pills also come in varying types, including varying doses of estrogen, and whether the dose of estrogen or progestogen changes from week to week.
Combination pills usually work by preventing a woman's ovaries from releasing eggs (ovulation). They also thicken the cervical mucus, which keeps sperm from joining with an egg. The hormones in combination and progestogen-only pills also thin the lining of the uterus. In theory, this could prevent pregnancy by interfering with implantation of a blastocyst.
Combined oral contraceptive pills
All contain an estrogen, ethinyl estradiol or mestranol (the inactive 3-methyl ether of ethinyl estradiol, which must be metabolized by the liver into the active ethinyl estradiol; 50 µg of mestranol is equivalent to only 35 µg of ethinyl estradiol and should not be used when high-dose (50 µg ethinyl estradiol) estrogen pills are needed; mestranol was the estrogen used in the first oral contraceptive, Enovid), although in varying amounts, and one of a number of different progestogens. They are usually taken for 21 days with then a 7 day gap during which a withdrawal bleed (often, but incorrectly, referred to as a menstrual period) occurs. These differ in the amount of estrogen given, and whether they are monophasic (only one dose of estrogen and progestogen during the 21 days) or multiphasic (varying doses). The introduction of extended-cycle monophasic pills (i.e. Seasonale) has shown that the menstrual intervals can be decreased.
These are typically given as 21 tablets of estrogen and progestogen, followed by 7 tablets of placebo or an iron supplement, although some newer formulations contain more active tablets and fewer placebos. Everyday regimens (Microgynon 30 ED, Femodene ED, Logynon ED), which include seven inactive placebo pills, are rarely used in UK practice. Different formulations contain different amounts of estrogen and progestogen:
Progestogen only pills (POPs) use progestogen alone with doses taken continuously and no gap between packs taken. Women may experience irregular light bleeds on POPs, and whilst irregular in the first few months of taking, usually settles to a regular pattern in time.
^ Hatcher, Robert A.; Nelson, Anita (2004). "Combined Hormonal Contraceptive Methods", in in Hatcher, Robert A. (ed.): Contraceptive Technology, 18th rev. ed., New York: Ardent Media, pp. 391-460. ISBN 0-9664902-5-8.
^ Speroff, Leon; Darney, Philip D. (2005). "Oral Contraception", A Clinical Guide for Contraception, 4th ed., Philadelphia: Lippincott Williams & Wilkins, pp. 21-138. ISBN 0-7817-6488-2.
^FFPRHC (2007). Clinical Guidance: First Prescription of Combined Oral Contraception. Retrieved on 2007-07-07.