Plan B (levonorgestrel) is estimated to be 60% to 94% effective at preventing pregnancy when taken within 3 days (72 hours) after unprotected sex. Somewhere between 0.6% to 2.6% of women who take Plan B within this time frame will still become pregnant.

While Plan B can be taken up to 72 hours after unprotected sex, its efficacy decreases with time, and it is best to take it as soon as possible. For example, the chances of pregnancy are higher if Plan B is taken 4 days after unprotected sex, but are subsequently lowered if taken 3 days, 2 days, 1 day or within 24 hours after unprotected sex. However, the American College of Obstetricians and Gynecologists notes that some studies suggest that it can be moderately effective at preventing pregnancy up to 5 days after unprotected sex. Five days after unprotected sex is the latest time with which Plan B has been shown to lower your chances of pregnancy.

The efficacy of Plan B is also affected by the time at which it is taken within your menstrual or reproductive cycle. Plan B works best when taken before ovulation, or when an ovary releases an egg. Once there is a surge in the hormone that initiates ovulation and ovulation begins, Plan B cannot prevent pregnancy. Its primary mechanism of action is to change hormone levels such that ovulation is delayed. This is why it is important to take Plan B as soon as possible after unprotected sex. As treatment is delayed, ovulation becomes more imminent (if it has not yet occurred).

A review of studies in Clinical Obstetrics and Gynecology showed that among 492 women who took Plan B after unprotected sex, those who took it on or after the day of ovulation had the same pregnancy rate as would be expected among women who used no contraception at all. By comparison, none of the women who took Plan B before ovulation became pregnant.

In the event of another unprotected sexual encounter after Plan B is taken, Plan B will not protect against pregnancy—you will need to take Plan B again after another unprotected sexual encounter.

Plan B may also be less effective at preventing pregnancy in women who are overweight or obese. Studies reviewed by Cochrane suggest that the dose of levonorgestrel in Plan B has a lowered potency when taken by women who have a higher body mass index.