Answers to Frequently Asked Questions About...
How to Use Emergency Contraception
What if I take the second dose of emergency contraceptive pills late?
The instructions for Plan B and
some other progestin-only
emergency contraceptive pills say that you should take two doses
12 hours apart. (If you’re using Plan B, that’s one pill
followed 12 hours later by another pill.) However, research shows
that Plan B and other progestin-only
emergency contraceptive pills (also called "morning
after pills" or "day after pills") are equally
effective if you take the two doses at the same time, take them 12
hours apart as recommended, or wait as long as 24 hours to take the
second dose. That’s why, in many countries, the instructions
for specially packaged progestin-only
emergency contraceptive pills like Plan
B say to take both pills at once. Some places even sell progestin-only
emergency contraception as a single pill that contains the same amount
of hormones found in two Plan B pills.
Research about “combined” emergency
contraceptive pills, in contrast, has focused on taking the two
doses 12 hours apart. Even so, taking the second dose a little early
or a little late (like a difference of two hours) will probably not
make a difference in how effective the pills are.
Click here for more detailed
instructions for using Plan B or
daily birth control pills for emergency contraception.
A thorough and up-to-date academic review of research about how and
when to take emergency contraceptive pills, as well as other medical
and social science literature about emergency contraception is available
here 
.