Plan B (levonorgestrel emergency contraception) is quite effective at reducing the chance of pregnancy after unprotected sex, especially if taken within 24 hours, but it is not 100% and works less well the longer you wait and around the time of ovulation. It should be seen as a backup method, not regular birth control.

What Plan B Is

Plan B One‑Step is a single‑dose emergency contraception pill that contains levonorgestrel, a progestin hormone used in many birth control pills. It is taken after sex to reduce the chance of pregnancy, not to end an existing pregnancy.

  • Used after: no condom, condom break, missed pills, or sexual assault.
  • Available: over the counter in many places without a prescription.
  • Not an abortion pill: it works before a pregnancy is established.

How Effective Plan B Is

Effectiveness depends heavily on timing, where you are in your cycle, your weight, and some medications.

  • Overall, Plan B can reduce the risk of pregnancy by about 75–89% if taken within 72 hours (3 days).
  • Clinical data show pregnancy rates roughly:
    • Within 24 hours: about 95% effective (very low pregnancy rate, around 0.4–1%).
* 25–48 hours: effectiveness drops to around **85%**.

* 49–72 hours: effectiveness around **58–61%**.
  • The manufacturer describes it as: about 7 out of 8 people who could have become pregnant do not get pregnant after taking it.

What Affects How Well It Works

Several factors can make Plan B more or less effective.

  • How fast you take it
    • Sooner is best; taking it within 12–24 hours gives the highest protection.
* It can be taken up to 72 hours for labeled use, and possibly up to 5 days, but effectiveness drops the longer you wait.
  • Ovulation timing
    • Plan B mainly works by delaying ovulation (preventing an egg from being released).
* If you already ovulated and the egg has been released and fertilized, Plan B is likely not effective, so pregnancy can still occur.
  • Body weight/BMI
    • Some research suggests Plan B may be less effective in people with higher body weights or BMI over ~30.
* However, it still offers some protection and major health sites advise not skipping it if you need emergency contraception.
  • Medications and supplements
    • Certain drugs can lower effectiveness, including:
      • Rifampin (antibiotic) and some HIV medicines
      • Some anti‑seizure medications
      • The herbal supplement St. John’s wort

What Plan B Does (and Doesn’t) Do

Understanding its limits helps set realistic expectations.

  • What it does :
    • Delays or prevents ovulation so that sperm and egg never meet.
* Modestly lowers the chance of pregnancy after a single episode of unprotected sex.
  • What it does not do:
    • Does not work reliably if taken after ovulation has already occurred.
* Does not terminate an existing pregnancy and is not an abortion method.

* Does not protect against STIs; it is only for pregnancy prevention.

* Is not meant to be regular birth control because it is less effective than ongoing contraceptives.

Side Effects and When To Test

Most people tolerate Plan B well, but short‑term side effects and follow‑up are important to know.

  • Common temporary side effects:
    • Nausea, fatigue, headache, breast tenderness, or dizziness.
* Changes in the next period: it may come earlier or later and may be lighter or heavier.
  • When to take a pregnancy test or see a doctor:
    • If your period is more than about 1 week late, take a pregnancy test.
* If you have very heavy bleeding, severe abdominal pain, or ongoing irregular bleeding, seek medical care to rule out problems like ectopic pregnancy.

Other Emergency Options

For some people, other emergency contraception methods may be more effective.

  • Ulipristal acetate (ella)
    • Works up to 5 days after sex with more consistent effectiveness throughout that window.
* May work better than Plan B for some people with higher body weight, though effectiveness can decrease above certain weights.

* Requires a prescription.
  • Copper IUD
    • The copper IUD is the most effective form of emergency contraception, with pregnancy rates under 0.1% when placed within 5 days of unprotected sex.
* It also provides long‑term contraception once inserted.

If there is any chance you might be pregnant after taking Plan B (for example, no period within 3 weeks or a positive pregnancy test), contacting a healthcare professional or clinic can help with safe next steps and support.