Answers to Frequently Asked Questions About...
Effectiveness
How effective is emergency contraception?
Emergency contraception makes it much less likely that you will get pregnant if taken within the first few days after you have sex. How much it reduces your chances of getting pregnant depends on which kind of emergency contraceptive you use and how quickly you take it after unprotected intercourse. In general, progestin-only emergency contraceptive pills, like Plan B, are more effective than combined emergency contraceptive pills.
Labeling for Plan B and other emergency
contraceptive pills (also known as "morning
after pills" or "day after pills") that contain
only the hormone progestin
states that the treatment reduces your risk of pregnancy by 89%*. This doesn’t mean that
11 percent of women will get pregnant using these pills. It just means
that this type of emergency
contraception prevents 89% of the pregnancies researchers would
expect to happen when women don’t use birth control, their regular
contraceptive fails, or they are forced
to have sex (in other words, they have “unprotected sex”).
Usually, if 100 women have unprotected sex one time during the second
or third week of their monthly menstrual cycle, 8 of them will get
pregnant. But if those same 100 women use Plan
B, only one will get pregnant. Put another way, this is an 89%
reduction in the expected number of pregnancies. And if you take these
progestin-only emergency
contraceptive pills within the first 24 hours after sex, they
reduce your risk of pregnancy by up to 95%.
Emergency contraceptive pills containing both progestin
and estrogen (known as “combined” pills) reduce the
risk of pregnancy by 75%. In other words, if 100 women use this type
of pill after having unprotected sex, only 2 of them will get pregnant.
Emergency insertion of a copper-T IUD reduces the risk of pregnancy
by more than 99%.
While both types of emergency
contraceptive pills are safe
and effective, they are not as good at preventing pregnancy as birth
control that’s used before or during sex, like the pill
or condom. If emergency contraception was the only type of birth control
you used for an entire year, your annual risk of getting pregnant
would probably be about 20% with Plan
B and other progestin-only emergency contraceptive pills and 40% with “combined” emergency contraceptive
pills. And that’s assuming you used emergency contraceptive
pills perfectly, after each and every time you had unprotected sex.
For this reason, health care providers generally recommend that, if
you are sexually active, you should try to find a regular method of
contraception other than emergency contraception that can work for
you. (Find out more about your options here.)
* The exact effectiveness of emergency contraceptive pills is difficult to measure and some researchers believe the effectiveness may be lower than that reported on package labels. To find out more about studies evaluating the effectiveness of emergency
contraception, read our thorough and up-to-date academic
review 
of the medical and social science literature here.