One-dose emergency contraception pills prevent pregnancy about 50-100% of the time. Some reasons emergency contraceptive pills can fail include ovulation timing, BMI and drug interactions.

How do you know if Plan B failed?

The only way to know if the morning after pill has been effective at preventing pregnancy is if your next period arrives when it should. The morning after pill works by delaying ovulation so that you don't release an egg for remaining sperm in your system to fertilize.

Can you still get pregnant after taking Plan B?

Bottom line—You can still get pregnant after taking Plan B

Also, if you take Plan B after unprotected sex and then have unprotected sex again, you'll need to take another pill. A long-term form of birth control is the best way to prevent pregnancy.