Oral contraceptives are contraceptives which are taken orally and inhibit the body's fertility by chemical means. Female oral contraceptives have been on the market since the early 1960s. Male oral contraceptives remain a subject of research and development, and are not available widely (if at all) to the public. Studies continue of various alternatives, such as gossypol.

Female oral contraceptives, colloquially known as the Pill, are the most common form of pharmaceutical contraception, the prevention of unwanted pregnancy. They 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, while others keep a constant level of the hormones. It is used by millions of women around the world, though the acceptance varies by region: approximately one-third of sexually active women in the United Kingdom use it, while in Japan, continual debates over safety and STD risks (raised over concerns that oral contraceptive use will diminish condom use) have led to the pill being banned for nearly 40 years, and its recent introduction has seen very few women take it up.

The Pill works by preventing ovulation, as well as making the uterus less likely to accept implantation of an embryo if one is created, and thickens the mucus in the cervix making it more difficult for sperm to reach any egg. Taken correctly, it is the single most reliable form of reversible contraception, with less than one in 100 women using the pill becoming pregnant in a year of continuous use.

Several different types of 'the pill' exist. Generally, they all have revolved around different synthetic estrogens and progestins, chemical analogues of the natural hormones estradiol (an estrogen) and progesterone (a progestagen). Most common brands use 20 to 40 micrograms of ethinyl estradiol as the estrogen component and either a fixed or varying (the bi and triphasic pills) amount of either levonorgestrel or norethindrone as the progestagen component.

The pill usually comes in two different packet sizes, and each packet usually has days marked off for a cycle lasting about a month, or 28 days to be exact. For the 21-pill packet, a woman takes a pill each day for 21 days, and waits for an additional seven days before starting the next packet. For the 28-pill packet, the woman similarly takes a pill each day. However, instead of only taking pills for 21 days of the month, she also takes the remaining seven placebo or sugar pills pills included in the packet, and once she finishes the last placebo pill, she can immediately start the next packet on the following day. The purpose of the placebo pills is to ensure that the woman, out of habit, can take a pill on every day of her menstrual cycle, so that she does not have to calculate when exactly is the next date that she should start her next packet of pills. It is possible for a woman to skip menstruation and still remain protected against conception by skipping these pills in the cycle. The presence of these pills is still thought to be comforting for the woman as menstruation is a physical confirmation that she is still not pregnant.

Some drugs reduce the effect of the pill and can cause breakthrough bleeding, or worse, pregnancy (together with unprotected sex, of course). These include antibiotics, barbiturates, phenytoin and carbamazepine. The traditional medicinal drug St John's Wort has also been implicated.

Serious side effects that would indicate a need to discontinue the pill include: pain or swelling in the thigh or calf, severe headaches, hypertension, dizziness, weakness, vision problems, chest pain or shortness of breath, abdominal pain.

These serious side effects are a result of the pill affecting coagulation, increasing the risk of blood clots causing DVTs (deep venous thrombosis), stroke and heart attacks. This is especially so in women who already have some pre-existing vascular disease, in women who have a familial tendency to form blood clots (such as familial factor V Leiden), women with hypercholesterolaemia (high cholesterol level) and in smokers.

When starting to take the birth control pill some people may experience slight weight gain, although this depends entirely on the individual and some people experience no weight gain at all while people who are already obese or overweight may continue to gain weight. Some people also notice changes in the intensity of sexual desire, vaginal discharge and menstrual flow. Some other common side effects are: nausea; headaches; depression; vaginitis; urinary tract infection; breast changes; skin problems; and gum inflammation.

The main causes of these are the way individuals react to the chemicals in certain types of pill.

The use of oral contraceptives has also been shown to have other side effects, such as a marked decrease in the rate of metabolism of caffeine, and changes in body-odor preferences for members of the opposite sex—women not on oral contraceptives tend to prefer the body odors of men with dissimilar major histocompatibility complexes (MHCs), whereas those on oral contraceptives tend to prefer those with similar MHCs. (This may help explain why some women fall in love while taking oral contraceptives, only to find that, after they get married and stop taking them, they no longer find their mate (as) attractive.)

The pill slightly increases the risk of breast cancer, while slightly decreasing the risk of ovarian cancer and uterine cancer. In rare cases, high estrogen pills may trigger pseudotumor cerebri, also known as benign intracranial hypertension.

Carl Djerassi (1923-) invented norethindrone in the 1950s..

The Pill's development in the early 1960s and FDA approval of the drug on May 9, 1960, was one of the major factors leading to the sexual revolution that occurred later in the decade.

The pill was rendered legal in 1967 after the introduction of the Neuwirth Law

• Morning-after pill
• Katharine McCormick

Hormonal contraceptive use in Japan (2004 news article)