The morning-after pill has been a little-known contraceptive secret. When the condom breaks, the diaphragm slips or passion takes over, doctors sometimes prescribe a megadose of birth-control pills to ensure those women don’t get pregnant.

In 1997, emergency contraceptives were approved by the FDA and are now available by prescription from Ob/Gyns, family-planning clinics, emergency rooms and HMOs. You can even get a prescription to keep around for emergencies. However, ”for the pills to work, timing is everything,” cautions Lisa Koonin, R.N., chief of reproductive health surveillance at the Center for Disease Control. ”The first dose must be taken within 72 hours, and the second dose 12 hours after that.”

The megadose of hormones prevents pregnancy by warding off implantation or ovulation, which doesn’t disrupt an existing pregnancy, like abortion. And the only side effects are some nausea or vomiting. But don’t try to self-medicate with a friend’s birth-control pills, and don’t use this emergency post-coital precaution as your primary contraceptive—too many hormones can upset your reproductive system for good. And if you’re already pregnant or you missed the three-day mark, it’s too late for the pills to work.