IStock Photo 4162109 © Simon Smith

It’s ten o’clock at night on a college campus. Why are a bunch of cell phone alarms going off?

Could be the daily reminder—programmed into phones by women all across the country—to pop a birth control pill, often called the candy of the campus. The odds a contraceptive-using woman in her early 20s is on the pill are 1 in 1.91 (52%).

But it’s not just young women who punch a pill through the foil casing each day: the odds any sexually active woman (15 - 44 years old) has ever taken the pill for contraceptive reasons are 1 in 1.22—that’s 82%. The birth control pill is an attractive contraceptive method for married women as well; condom use declines as women get older and marry, and many of these married women (1 in 4.24, in fact) choose the pill.

With so many women using birth control pills at some point in their lives, researchers have turned a more critical eye to the effects of the hormones contained within the daily dose. All pills involve progestin or a combination of progestin and estrogen. When a female body is exposed to a low but constant estrogen level supplied by the pill, the estrogen convinces the body that it is pregnant, thus preventing the release of chemicals that spark ovulation. Progestin, on the other hand, works by inducing the body to form its own barrier of cervical mucus, blocking sperm from traveling into the uterus.

These hormones provide reliable birth control yet come with baggage—everything from annoying side effects such as bloating and nausea to potential long-term health consequences such as cardiovascular problems, clogged arteries, bone loss, and depression. As for cancer risks and the pill, it’s a mixed bag: studies have shown that the pill raises your risk of breast, cervical, and liver cancer, yet research has also shown that it reduces risks of ovarian and endometrial cancer. A 2007 article in the British Medical Journal suggested birth control pill use lowered a woman’s overall risk of cancer by as much as 12 percent.

Other vehicles for delivering hormonal contraception are gaining in popularity. The odds a contraceptive-using woman gets the Depo-Provera shot, for instance, are 1 in 18.87. And researchers haven’t been sitting still; in 2009, the European drug regulatory agency approved a new morning after pill, ulipristal acetate (sold as ellaOne in Europe), which a recent study found to be more effective than previous such drugs. With so many birth control options, women who want to understand the benefits and risks might want some help; the odds a woman will receive birth control counseling in a year are 1 in 5.38. However, that’s less than half as likely that a woman 18 - 24 has some college education or an associate's degree (1 in 2.27). Maybe that helps explain all those charged cell phone batteries, ready for that 10 PM pill alert.