These differ in the amount of estrogen given, and whether they are monophasic (the same dose of estrogen and progestogen during each of the 21 days) or multiphasic (varying doses). The introduction of extended-cycle monophasic pills (i.e. Seasonale) has shown that the withdrawal bleeding intervals can be decreased. Monophasic
These are typically given as 21 tablets of estrogen and progestogen, followed by seven 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:

15 μg ethinylestradiol
60 μg gestodene: 24 days + 4 days placebo (Spain: Melodene-15; Israel: Minesse)
20 μg ethinylestradiol
1000 μg norethisterone acetate (UK: Loestrin 20, Galen; US: Loestrin 1/20, Duramed; Microgestin 1/20, Watson Pharmaceuticals; Junel 1/20, Barr)
1000 μg norethisterone acetate: 24 days + 4 days ferrous fumarate only (US: Loestrin 24 Fe, Warner Chilcott)
90 μg levonorgestrel: continuous: 365 days/year, no placebo (US: Amethyst, Watson)
100 μg levonorgestrel: extended cycle: 84 days + 7 days 10 μg ethinylestradiol only (US: LoSeasonique, Teva; CamreseLo, Teva)
100 μg levonorgestrel (US: Alesse, Wyeth; Aviane, Barr; Lessina, Barr; Lutera, Watson; Sronyx, Watson)
150 μg desogestrel (UK: Mercilon, Organon; HU: Novynette, Richter Gedeon)
75 μg gestodene (UK: Femodette, Bayer; RU: Logest, Bayer; Brazil: Femiane, Bayer)
3000 μg drospirenone: 24 days + 4 days placebo (US, EU, RU: Yaz; Bayer Schering Pharma AG.
Birth control pills come in a variety of formulations. The main division is between combined oral contraceptive pills, containing both estrogens and synthetic progestogens (progestins), and progestogen only 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. Mechanism of action
Combination pills usually work by preventing the ovaries from releasing eggs (ovulation). They also thicken the cervical mucus, which keeps sperm from penetrating into the uterus and joining with an egg. The hormones in combination and progestogen-only pills also thin the lining of the uterus. This could prevent pregnancy by interfering with implantation of a blastocyst. Main action in typical use is prevention of ovulation. Combined oral contraceptive pills
All contain an estrogen, ethinylestradiol or mestranol, in varying amounts, and one of a number of different progestogens. (Regarding the estrogen, the inactive 3-methyl ether of ethinylestradiol, which must be metabolized by the liver into the active ethinylestradiol; 50 μg of mestranol is equivalent to only 35 μg of ethinylestradiol and should not be used when high-dose [50 μg ethinylestradiol] estrogen pills are needed; mestranol was the estrogen used in the first oral contraceptive, Enovid). They are usually taken for 21 days with then a seven-day gap during which a withdrawal bleed (often, but incorrectly, referred to as a menstrual period) occurs.