The combined oral contraceptive pill (OCP), often referred to as "the Pill", is a combination of an oestrogen and a progestin (progestogen), taken by mouth to inhibit normal fertility.

Combined oral contraceptive pills must be ingested at the same time each day. Women must take the pill within a two hour window each day to ensure its effectiveness. Most brands of combined pills are packaged in one of two different packet sizes, with days marked off for a 28 day cycle. For the 21-pill packet, a pill is consumed daily for three weeks, followed by a week of no pills. For the 28-pill packet, 21 pills are taken, followed by week of placebo or sugar pills. A woman on the pill will have a withdrawal bleed sometime during the placebo week.

If a woman just starting the pill begins taking them within five days of the beginning of her menstrual cycle (the menstrual cycle begins on the 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.

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. Diarrhoea can also stop the Pill from working, because it causes the hormones to not be properly absorbed by the bowels.

The combined Pill primarily prevents pregnancy by preventing ovulation. In women who take the pills correctly, ovulation is prevented in 98-99% of cycles.

Secondary mechanisms active before fertilization include thickening of cervical mucus (which can prevent or slow sperm entry into the uterus) and changes in tube transport, inhibiting sperm migration to the fallopian tubes.

Drug interactions

Some drugs reduce the effect of the Pill and can cause breakthrough bleeding, or increased chance of pregnancy. These include drugs such as rifampicin, barbiturates, phenytoin and carbamazepine.

In addition, cautions are given about broad spectrum antibiotics, such as ampicillin and doxycycline, which may decrease the effectiveness of the OCP. While taking these antibiotics, alternative contraception should be used.

The traditional medicinal herb St John's Wort has also been implicated due to its upregulation of the P450 system in the liver.


Different studies have found different risks of side effects. Some sources have found that the majority (about 60%) of women report no side effects at all, and the vast majority of those who do, have only minor effects. Other studies have found that up to half of women who try the pill discontinue due to side effects.

There is no evidence that modern low-dose pills cause weight gain. However, fear of weight gain can contribute to poor compliance in taking the Pill and subsequent unintended pregnancy, especially among adolescents.

Possible side effects include: vaginal discharge, changes in menstrual flow, breakthrough bleeding, unusual build-up of the uterine lining, nausea, headaches, depression, vaginitis, urinary tract infection, changes in the breasts, changes in blood pressure, skin problems, skin improvements, and gum inflammation.

Cautions and contraindications

Oral contraceptives may influence coagulation, increasing the risk of deep venous thrombosis (DVT) and pulmonary embolism (PE), stroke and heart attack.

Combined oral contraceptives are generally accepted to be contraindicated in women with pre-existing cardiovascular disease, in women who have a familial tendency to form blood clots (such as familial factor V Leiden), women with severe obesity and/or hypercholesterolaemia (high cholesterol level), and in smokers over age 35.

There is a small increase in the relative risk of breast cancer while taking the OCP.

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