What Are the Chances of Plan B Failing?

Plan B emergency contraception works by temporarily delaying the release of an egg from the ovary, so there’s no egg to meet the sperm and thus no pregnancy can occur.

Plan B One-Step (levonorgestrel) is a brand of birth control used in unanticipated or emergency situations as a back-up method to prevent pregnancy. It may sometimes be called “the morning-after pill,” “emergency contraception,” or “postcoital contraception.”

Plan B emergency contraception works by temporarily delaying the release of an egg from the ovary, so there’s no egg to meet the sperm and thus no pregnancy can occur. 

Other types of emergency oral contraception include: 

• Levonorgestrel (AfterPill, My Way, Next Choice One Dose, and Take Action): available over-the-counter (OTC)
• Ulipristal acetate (ella, ellaOne): available by prescription only

Plan B One-Step emergency oral contraception is not the same as “the abortion pill,” which is a two-pill regimen of different medications, mifepristone, and misoprostol. If you are already pregnant, Plan B will not harm a fetus or interfere with a pregnancy.

Plan B One-Step is intended to be taken within 72 hours after unprotected sex. The sooner it is taken, the better. 

Plan B is not intended to be used as a primary method of birth control. It is intended to be used after sex to prevent pregnancy in the following situations:

• Birth control was not used 
• Birth control that was used did not work or was not used properly
  • The male condom broke or slipped off during intercourse
  • One or more doses of hormonal birth control pills were missed
  • Birth control patch was applied too late or removed too soon, or it did not stick well to the skin 
  • The vaginal ring was inserted too late or was removed too soon during the month
  • Diaphragm with spermicide moved out of place, had a tear/hole, or was not used with spermicide
  • The cervical cap slipped off, had a tear/hole, or was not used with spermicide
  • Intrauterine device (IUD) came out before it was ready to be removed
  • Birth control injections were received more than two weeks late 
  • Implantable rod hormone device was not removed or replaced in time
  • Spermicide-killing tablet or film did not melt before sexual intercourse
  • A man failed to withdraw in time and ejaculated inside the woman or on her genitals
  • A woman who used the rhythm method did not correctly estimate the “safe” time in her cycle or had unprotected intercourse during a fertile time 

When women take Plan B as directed, about 7 out of every 8 women who could have gotten pregnant will not become pregnant after taking Plan B. 

There are several factors that influence how effective the Plan B will be, including:

• Where a woman is in her menstrual cycle
• How soon after unprotected sex Plan B is used
  • Plan B is intended to be taken within 72 hours after unprotected sex, but the sooner it’s used, the more effective it will be
• The use of certain drugs may decrease the effectiveness of Plan B 
  • Barbiturates, bosentan, carbamazepine, felbamate, griseofulvin, oxcarbazepine, phenytoin, rifampin, St. John’s wort, topiramate, and certain HIV/AIDS medications (such as protease inhibitors and non-nucleoside reverse transcriptase inhibitors) 

Side effects of Plan B, when used right after unprotected intercourse, are usually mild and include:

• Next menstrual period that may be early or late, lighter or heavier
• Nausea 
• Vomiting
• Lower abdominal pain/cramps
• Headache
• Tiredness/fatigue
• Dizziness
• Breast tenderness or pain
• Spotting between periods