When you take Plan B (the “morning‑after pill”), you get a short blast of hormone that delays ovulation so sperm never meet an egg, which lowers your chance of getting pregnant after unprotected sex.

What Plan B Actually Does

  • Plan B contains a high dose of levonorgestrel, a synthetic form of progesterone.
  • Its main job is to temporarily delay ovulation (the release of an egg).
  • If there’s no egg released, sperm have nothing to fertilize, so pregnancy is much less likely.
  • It works best the sooner you take it and is most effective within 72 hours after sex.
  • It does not work if you’ve already ovulated and it does not end an existing pregnancy (so it’s not an abortion pill).

Think of it as hitting “pause” on ovulation for a short time so the window for fertilization closes.

What You Might Feel: Common Side Effects

Many people feel totally fine; others feel off for a day or two.

Typical short‑term effects can include:

  • Nausea or vomiting
  • Tiredness or feeling “washed out”
  • Headache
  • Dizziness
  • Breast tenderness
  • Lower belly (crampy) pain
  • Spotting or light bleeding a few days later
  • A period that’s a bit earlier, later, heavier, or lighter than usual

These effects are usually mild and pass on their own within a few days, and any cycle changes are usually limited to the next period.

What Happens to Your Period

  • Your next period might come a little early or a few days late.
  • It might be heavier than usual or come with more cramping.
  • Some people notice light spotting before their actual period arrives.

If your period is more than about a week late after taking Plan B, it’s smart to take a pregnancy test and/or talk to a clinician.

When to Worry and Call a Doctor

Most people don’t have serious problems, but you should get medical help if you notice:

  • Very heavy bleeding (soaking a pad or tampon every hour for several hours)
  • Severe belly or pelvic pain (especially on one side)
  • Ongoing, intense nausea, vomiting, dizziness, or feeling very unwell

Severe pain or unusual bleeding can, rarely, signal an ectopic pregnancy (a pregnancy outside the uterus), which needs urgent care.

If you throw up within about an hour of taking Plan B, you may need another dose because your body might not have absorbed it.

What Plan B Does Not Do

  • It is not meant to be used as regular birth control; it’s less effective than methods like the pill, IUD, or implant.
  • It does not protect against STIs.
  • It does not make you infertile or harm your ability to get pregnant in the future.
  • It only helps with the specific incident of unprotected sex you took it for; it doesn’t protect you for sex that happens afterward.

Quick HTML Table Overview

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What happens when you take Plan B Details
Main action High- dose levonorgestrel temporarily delays ovulation so no egg is available to be fertilized.
Best timing Take as soon as possible, up to 72 hours after unprotected sex; earlier is more effective.
Effectiveness Can reduce risk of pregnancy by up to about 89% when taken correctly, but is less effective than regular contraception.
Common side effects Nausea, tiredness, headache, dizziness, breast tenderness, abdominal pain, spotting, and period changes, usually short term.
Period changes Next period may be earlier or later and may be heavier or lighter than usual; spotting can occur.
Serious symptoms (rare) Severe pain or heavy bleeding may signal a problem like ectopic pregnancy – seek medical care.
What it is not Not for regular birth control, not an abortion pill, no STI protection, doesn’t harm future fertility.

Little “Story” Example

Imagine someone has a condom break on a Saturday night and takes Plan B the next morning. Over the next day she feels a bit tired and slightly nauseated, then notices some spotting mid‑week and her period shows up a few days later than usual but heavier than normal. She doesn’t have severe pain or very heavy bleeding, so she just keeps an eye on her cycle and uses this as a prompt to talk with a provider about a more reliable “Plan A” birth control method.

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Learn what happens when you take Plan B: how it works, side effects, period changes, timing, and when to see a doctor, plus why it’s for emergencies only and not regular birth control.

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