How Effective Is the Plan B Pill?

By Carla Boulianne

Plan B pills are an emergency contraceptive option for women after unprotected intercourse due to primary contraceptive failure or other circumstances. The woman takes a tablet containing 0.75 mg of levonorgestrel, a synthetic progestin, up to seventy-two hours after intercourse. She takes a second dose twelve hours later. Efficacy depends on the timing of the first dose, with earliest administration after intercourse providing the greatest protection from unintended pregnancy.


Although the exact mechanism of pregnancy prevention will vary depending on the day of the woman’s cycle, hormonal contraceptives like Plan B inhibit ovulation, reduce sperm motility, and effect ovum transport through the fallopian tubes. Plan B is ineffective after implantation of a fertilized egg.

Time Frame

Plan B is most effective when taken within 72 hours of unprotected intercourse. Plan B is effective in reducing one-time pregnancy rates by more than 75% for the average user. Prompt initiation of treatment substantially improves that rate. The relationship between time of first dose and pregnancy rate is linear; Plan B is most effective when taken immediately after intercourse.

Estimated from research by Piaggio, the pregnancy rate is less than 1% (about 0.5%) when the delay in taking the first dose is only zero to twelve hours. By comparison, taking the first dose of Plan B two days after unprotected intercourse carries 3 to 4 times the failure rate. Starting Plan B three days after intercourse translates to a 4% risk of pregnancy.

When taken as directed, the cumulative efficacy for all users starting Plan B within 72 hours of intercourse is as high as 89%. The majority of women experiencing emergency contraceptive failure started Plan B pills more than two days after unprotected intercourse.


Due to the substantial increase in efficacy seen when taking Plan B immediately after unprotected intercourse, reproductive health providers encourage women to obtain an advance supply of Plan B emergency contraceptive.


Taking Plan B up to 120 hours after intercourse still provides significant protection, but the anticipated pregnancy rates are higher. Taking both doses simultaneously or taking the second dose sooner than twelve hours after the first Plan B dose does not reduce efficacy. Planned Parenthood has already incorporated this recent research into their Plan B pill use recommendations.


A study by the Alan Guttmacher Institute exploring an 11% decline in United States abortion rates between 1994 and 2000 found that increased availability of emergency contraceptives contributed substantially to the decrease in elective abortion rates. Since the 2006 approval of Plan B for over-the-counter purchase, sales of Plan B doubled to nearly three million doses annually by 2008. The United States has not yet realized the full impact of easily accessible emergency contraceptives on unintended pregnancy rates.A large randomized control study of women in China showing no decrease in abortion rates after advance provision of emergency contraceptives did not use a levonorgestrel regimen like Plan B. The mifepristone (RU-486) supplied to the Chinese women is an effective abortifacient during the first 7 weeks of pregnancy, although the function is different preceding conception. The mechanism of action and side effect profile of mifepristone differ substantially from the Plan B emergency contraceptive, making this study a weak comparison and confusing the moral debate surrounding Plan B. Unlike mifepristone, Plan B does not cause abortion.