Principles

Female oral contraceptives, colloquially known as the Pill, are the most common form of pharmaceutical contraception. They are used to prevent pregnancy. The pill is also used for certain medical conditions, in particular: to control symptoms of polycystic ovary syndrome (PCOS), irregular or painful menstrual cycles (dysmenorrhea), anemia related to menstruation, and certain disorders where there is dysfunctional uterine bleeding, as well as situations of endometriosis.

Female oral contraceptives consist of a pill that women take daily and that contains doses of synthetic hormones (always a progestin and most often also an estrogen). In some types of pill the doses of hormones are adjusted to be in synchrony with the menstrual cycle (two- or three-phase pills), while others keep a constant level of the hormones.

Use
The Pill must be ingested daily, within 12 hours of the same time each day (for progesterone/estrogen pills — as opposed to within 3 hours for Progesterone only pills). Most brands are packaged with 21 days of active (hormone-containing) pills followed by either 7 days of placebo pills, or instructions to not take pills for seven days. A woman on the pill will have a withdrawal bleed, or period, sometime during the placebo week.

If a woman just starting the pill begins taking them on the first day of her menstrual cycle (first day of red bleeding), she will have pregnancy protection from the very first pill. If a woman begins taking the pill at another time in her menstrual cycle, she must use a different form of contraception for seven days.

Effectiveness
The Pearl Index is often used to compare the effectiveness of various methods of contraception. It is expressed as the "number of pregnancies in 100 normally fertile women over the period of one year". Each method of birth control has two Pearl index numbers:

method effectiveness: is the Pearl index number for use under perfect conditions. The method effectiveness Pearl index for the Pill has been measured as low as 0.3 and as high as 1.25, which means that under ideal conditions, anywhere from 0.3 to 1.25 out of 100 users will become pregnant during one year of perfect use (Pearl index = 0.3 to 1.25).
user effectiveness or typical effectiveness: is the Pearl index number for use that is not consistent or always correct. The user effectiveness measured by the Pearl index for the Pill has been measured as low as 2.15 and as high as 8.0, which means that anywhere from 2.15 to 8.0 out of 100 women will become pregnant during the first year of typical use (Pearl index = 2.15 to 8.0).
Many women occasionally forget to take the Pill daily, impairing its effectiveness. Correct use of the pill usually implies taking it every day at the same hour for 21 days, followed by a pause of seven days.

Use of other medications can prevent the Pill from working, due to interactions with the metabolism of the hormonal constituents. Diarrhea can also stop the Pill from working, because it causes the hormones to not be properly absorbed by the bowels.

While the Pill is usually effective, its wide availability has not prevented all unplanned pregnancies.

Packaging
The Pill usually comes in two different packet sizes, where each usually has days marked off for a cycle lasting of 28 days. For the 21-pill packet, a pill is consumed daily for three weeks, followed by one week of waiting period before starting the next packet. For the 28-pill packet, pills are consumed daily as well. However, instead of taking pills for 21 days only, the user also takes seven placebo or sugar pills included. Once they are finished, the next packet can be started right away.

The purpose of the placebo pills is that the user, out of habit, can take a pill on every day of her menstrual cycle, instead of calculating the date she should start the next dose. As these are placebos, failure to take them has no effect on the effectiveness of the Pill provided the regular schedule is followed. If the pill formulation is monophasic, it is possible to skip menstruation and still remain protected against conception by skipping the placebo pills and starting directly with the next packet. Attempting this with bi- or tri-phasic pill formulations carries an increased risk of breakthrough bleeding and may be undesirable. It will not, however, increase the risk of getting pregnant. The presence of placebo pills is thought to be comforting, as menstruation is a physical confirmation of not being pregnant. Breakthrough bleeding also becomes a more common side effect as a woman attempts to go longer periods of time between menstrual periods. The pills may contain an iron supplement, as iron requirements increase during menstruation.

Drug interactions
Some drugs reduce the effect of the Pill and can cause breakthrough bleeding, or increased chance of pregnancy. These include antibiotics, barbiturates, phenytoin and carbamazepine. The traditional medicinal herb St John's Wort has also been implicated.

Side-effects
When starting to take the Pill some women report slight weight gain, although most studies show that the incidences of this is about 50% and as many women experience slight weight loss. Some women also notice changes in the intensity of sexual desire, vaginal discharge and menstrual flow.

Other possible side effects are: breakthrough bleeding, nausea, headaches, depression, vaginitis, urinary tract infection, changes in the breasts, changes in blood pressure, skin problems, skin improvements, and gum inflammation. The insert included with each pill packet usually has a more extensive list of recognized side effects.