The common term for the female oral contraceptive pill (OCP) also known as the birth control pill (bcp)
Several flavours of the OCP exist. Generally, however, they all have revolve around different formulations of (chemical analogues of) the hormones progesterone and estrogen and work by fooling the woman's body into not ovulating. Most brands nowadays use 20 to 40 micrograms of ethinyloestradiol as the estrogen component and either a fixed or varying (the bi and triphasic pills) amount of progestogen as the progesterone analogue.
The pill usually comes in a packet that has days marked off for a cycle lasting about a month. There are usually sugar pills between the end of one cycle and the start of the next so that the woman's body is fooled into undergoing menstruation. That's the hormones taking over!
Taken correctly, the failure rate of the pill is less than 1%. It is the single most effective form of reversible contraception.
A commonly quoted side effect is weight gain. Other common side effects include:
nausea, headaches, depression, change in intensity of sexual desire and response, vaginitis and vaginal discharge, urinary tract infection, changes in menstrual flow, breast changes, skin problems and gum inflammation
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.
update #4: 6th April 2000 (original writeup started 1st April 2000) -- I really should start breaking this up into sections, huh?
The pill slightly increases the risk of breast cancer, while slightly decreasing the risk of ovarian cancer and uterine cancer. To understand why this is so, consider how in the pre-OCP pill, pre-modern society days, when women would get pregnant not long after menarche and continue having babies every 2 to 4 years thereafter, that that was the normal cycle and long periods of unpregnancy are actually abnormal in the evolutionary sense. Think about it.
update #5: Drug interactions of the oral contraceptive pill
Some drugs, when taken, reduce the effect of the pill and can cause breakthrough bleeding, or worse, pregnancy (together with unprotected sex, of course). The usual suspects are antibiotics, barbiturates, phenytoin and carbamazepine.