This type of pill contains estrogens and progestagens. Progestagens are mainly responsible for the effectiveness of the pill; the estrogens regulate the vaginal bleeding pattern. Combined OCs prevents pregnancy by inhibiting ovulation. Most combined OCs have to be taken in the following regimen: one tablet daily for 21 days followed by a 7 day pill-free interval. This is called the 21/7 regimen. During the 7 day pill-free interval a withdrawal bleeding occurs.
Some combined OCs should be taken for 22 consecutive days followed by a 6 day pill-free interval: the 22/6 regimen.
Another option is to take a placebo tablet (tablet without hormones) on the days on which usually no tablet would be taken; the 28/0 regimen.
This type of pill contains progestagens and is suitable for women who cannot or do not want to take estrogens. Most POPs prevent pregnancy by making the uterus difficult to enter for the sperm cells. This principle is less reliable than ovulation inhibition; therefore POPs are less reliable than COCs. In addition, POPs must be taken daily without a delay of more than 3 hours.
Recently a new type of POP has become available. This POP is called Cerazette and it is different from the traditional POPs as described above. Just like a combined OC, Cerazette inhibits ovulation. Because the working mechanism of Cerazette is comparable to that of a combined OC, it allows for a delayed intake of 12 hours instead of 3 hours.