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Oral contraceptives are classified on the basis of their estrogen and progestin content (see Table).

COMBINED ESTROGEN AND PROGESTIN PREPARATIONS (combined oral contraceptives-COCs-and other combination hormonal products) suppress ovulation by imitating the feedback inhibition of endogenous estrogen and progesterone on the pituitary and hypothalamus. This reduces the secretion of follicle-stimulating hormone and luteinizing hormone (which are necessary for growth and maturation of the follicle and for ovulation). These combinations also cause the endometrium to become thin and the cervical mucus to become resistant to sperm penetration. Tubal motility and secretion are also affected.
Most combination, biphasic, and triphasic oral contraceptives are available in both 21-tablet packs and 28-tablet packs. In the 21-tablet packs, all 21 tablets contain active ingredients. After the last pill in a pack is taken, a 7 day tablet-free period is followed by initiation of the next pack. In the 28-tablet packs, the first 21 or 24 pills contain active ingredients and the remaining pills contain inactive ingredients. No tablet-free days elapse between taking the 28^th tablet of one pack and the first tablet of the next pack. Menses usually occurs during the tablet-free or inert tablet days; the next pack should be started whether or not menses has occurred. Tablets should be taken at the same time each day for maximum efficacy, and should be taken in the correct order. Some products (eg, Mircette, Kariva), use two inert tablets between an initial 21-day phase combining a progestin with an estrogen and a second 5-day phase that consists of an estrogen alone. Seasonale is an extended-cycle oral contraceptive that consists of an 84-day cycle of active tablets followed by a 7-day cycle of inert tablets.
Several different regimens are recommended for beginning the first cycle of the various pill types, including Day 1 and Sunday starts. Day 1 is the first day of menses. In the Day 1 start regimen, the first pill of the first cycle is taken on the day menses begins. In the Sunday start regimen, the first pill is taken on the first Sunday after menses begins; if menses begins on a Sunday, then the first pill is taken that day. The Sunday-start regimen necessitates back-up contraceptive measures during the first cycle. In the event of a missed pill, the product labeling should be consulted; additional contraception may be needed until a normal menses begins, ensuring contraception for that cycle.

PROGESTIN-ONLY oral contraceptives, or progestin-only pills (POPs, or "mini-pills") alter cervical mucus, exert a progestational effect on the endometrium, interfering with implantation, and, in some patients, suppress ovulation. These pills are taken daily, every day, without a drug-free interval. It is particularly important that these be taken at the same time every day (preferably 4 to 6 hours before expected intercourse) and that back-up contraceptive measures are taken in the event of even one missed or late pill.