How Effective Is Plan B? Morning After Pill

Medically Reviewed on 3/2/2022
How Effective Is Plan B
Plan B is almost 95% effective at preventing pregnancy if taken within 24 hours of unprotected sex, and 87% effective if taken within 72 hours

Plan B, or the morning after pill, is almost 95% effective at preventing preganncy if taken within 24 hours of unprotected sex, and 87% effective if taken within 72 hours. In short, the sooner you take it, the more effective it is. 

Though emergency contraception is safe and effective, it should not be used as a replacement for regular contraception. As the name suggests, Plan B is a backup plan for emergencies and not recommended for routine use.

What is Plan B?

Plan B is an FDA-approved drug that contains levonorgestrel, a synthetic version of the progesterone hormone, which can be used for emergency contraception to prevent pregnancy after unprotected sex.

The pill contains 1.5 mg of levonorgestrel and is typically taken in a single dose regimen, although some brands require taking two pills are taken separately within 72 hours of unprotected sex.

Various brands available include:

  • Econtra EZ
  • My Way
  • Next Choice One Dose
  • Plan B One Step
  • Preventeza
  • Take Action
  • Option 2
  • AfterPill
  • My Choice
  • Aftera

How does Plan B work?

In a normal 28-day menstrual cycle, the following series of events occur:

  • Ovulation: A mature egg from the ovary is released, which occurs about day 15.
  • Fertilization: The egg is fertilized when sperm is released into the vagina through intercourse.
  • Implantation: The resulting embryo travels to the uterus (womb) and implants itself in the uterine wall, which occurs 6-9 days after fertilization.
  • Human chorionic gonadotropin (HCG) production: Tiny amounts of the pregnancy hormone HCG are produced by the cells that later form the placenta (an organ that provides nourishment to the growing fetus) and appear in urine, about 10 days after conception.

Levonorgestrel helps prevent pregnancy by:

  • Preventing or delaying ovulation
  • Preventing fertilization
  • Preventing implantation by thinning the uterus lining

Plan B pills can be purchased over-the-counter without a prescription in the following situations:

How often does Plan B fail?

About 7 out of 8 women can prevent unwanted pregnancy if Plan B is taken as directed. However, Plan B or emergency contraception may not work if you:


Which of the following are methods for contraception? See Answer

When should you not take Plan B?

Levonorgestrel should not be used:

What are the potential side effects of Plan B?

Side effects of Plan B are usually mild and often resolve on their own and may include:

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Medically Reviewed on 3/2/2022
Image Source: iStock Images

Stuart A. Levonorgestrel Emergency Contraception. WebMD.

Planned Parenthood. What’s the plan B morning-after pill?