How Effective Is the Morning After Pill?

Reviewed on 10/7/2021

What Is the Morning After Pill?

Emergency contraception or
Emergency contraception or "the morning after pill" or "Plan B" works by preventing fertilization of the egg.

Emergency oral contraception, often referred to as “the morning after pill,” “Plan B,” and “postcoital contraception,” is a type of birth control used in unexpected or emergency situations as a back-up method to prevent pregnancy

Types of emergency oral contraception include: 

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

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, the morning after pill will not harm a fetus or interfere with a pregnancy.

How Does Plan B (Morning After Pill) Work?

When Is the Morning After Pill Used?

The morning after pill may be taken any time during the menstrual cycle to prevent pregnancy. 

The morning after pill is not intended to be used as a primary method of birth control. It is usually 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
    • Male condom breaks or comes off during intercourse
    • Hormonal birth control pills are not taken every day as directed
    • Birth control patch is applied too late or it is removed too soon during the month, or it does not properly stick to the skin 
    • Vaginal ring is inserted too late or it is removed too soon during the month
    • Cervical cap slips off, has a tear/hole, or is not used with spermicide
    • Diaphragm with spermicide slips out of place, has a tear/hole, or is not used with spermicide
    • Intrauterine device (IUD) comes out before it is ready to be removed
    • Implantable rod is not removed or replaced in time
    • Birth control injections are received more than two weeks late 
    • Spermicide-killing tablet or film does not melt before sexual intercourse
    • A man fails to withdraw in time and ejaculates inside the woman or on her genitals
    • A woman who uses the rhythm method does not correctly estimate the “safe” time in her cycle or has unprotected intercourse during a fertile time 

How Effective Is the Morning After Pill?

There several factors that influence how effective the morning after pill (emergency contraception) will be, including:

  • Where a woman is in her menstrual cycle
  • How soon after unprotected sex emergency contraception is used
    • Emergency contraception works best when uses as soon as possible after unprotected sex 
    • Levonorgestrel pills and ulipristal acetate are most effective when taken as soon as possible after unprotected sex, but may be taken up to 5 days after sex
    • The treatment becomes less effective as more time goes by
  • The type of emergency contraception used
  • A woman’s weight
    • Emergency contraceptive pills are less effective in overweight and obese women 
    • If overweight women use oral emergency contraception, ulipristal seems to be more effective than levonorgestrel

A woman can still get pregnant if she has unprotected sex again after taking the morning after pill.

What Are Side Effects of the Morning After Pill?

Side effects of the morning after pill, when used right after unprotected intercourse, are usually mild and include:

Reviewed on 10/7/2021