Levonorgestrel emergency contraception: how it works, how to take it
Last updated September 3, 2026.
Levonorgestrel emergency contraception is a progestin tablet that prevents pregnancy after unprotected sex - it works by preventing the release of an egg from the ovary. You may know it as Plan B One-Step. A backup, not a routine method.
What does it treat?
Pregnancy prevention after unprotected sexual intercourse - when no birth control was used, or a method failed or was used incorrectly, like a condom that slipped or pills taken off schedule. It is not for regular use, and it does not protect against HIV or other sexually transmitted infections.
How do you take it?
As soon as possible: the single-tablet product, one tablet within 72 hours of unprotected sex - the sooner the better; the two-tablet product, one tablet as soon as possible within 72 hours and the second 12 hours later. If you vomit within 2 hours of a dose, call your doctor - another dose may be needed. It can be taken at any point in the cycle. If your period is more than a week late afterward, take a pregnancy test.
Side effects to know
- Common: heavier or lighter than usual bleeding, spotting between periods, nausea, vomiting, diarrhea, tiredness, headache, dizziness, and breast pain or tenderness.
- Serious - call your doctor immediately: severe lower abdominal pain 3 to 5 weeks after taking it - the ectopic-pregnancy signal.
Who should not take it?
Anyone already pregnant - it will not work and is not intended for that. It prevents ovulation; it does not end an established pregnancy. For regular contraception, the ongoing methods are the right tools - this is the backup.
When is it an emergency?
Severe lower abdominal pain in the weeks after taking it is urgent care - an ectopic pregnancy is dangerous and needs same-day evaluation. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
How does it work - and how fast do I need to take it?
It prevents the ovary from releasing an egg - no egg, no pregnancy. Timing is everything: within 72 hours of unprotected sex, and sooner beats later - effectiveness falls as hours pass. The single-tablet version is one dose; the two-tablet version splits it 12 hours apart. It is a backup for when the plan failed, not a standing method.
Will it work if I am already pregnant?
No - it prevents ovulation and does not affect an established pregnancy. It is not an abortion pill. If a period runs more than a week late after taking it, take a pregnancy test. And severe lower abdominal pain 3 to 5 weeks after taking it is a call-your-doctor-immediately symptom - that is the ectopic-pregnancy warning.
Why did I feel nauseous - and what if I vomit?
Nausea is the most common after-effect and fades within a day or two. The rule that matters: vomiting within 2 hours of a dose may mean the tablet never absorbed - call your doctor, because another dose may be needed. After the 2-hour mark, the dose is in.
What will it do to my next period?
Expect it to be different: heavier or lighter than usual, spotting before it, or a shift in timing - all listed, all normal. The outer marker: a period more than a week late means take a pregnancy test. The cycle typically normalizes the following month.
Can I use it as my regular birth control?
It is not designed for that - less reliable than ongoing methods and harder on your cycle with repeated use. If you are reaching for it more than once in a while, that is the signal for a real contraception conversation: pills, patch, ring, IUD, implant - the methods built for every day are all more effective than the backup used repeatedly.
Does it protect against infections?
No - it addresses pregnancy only, not HIV or other sexually transmitted infections. If the unprotected sex carried infection risk, testing is a separate, worthwhile step - and future protection is the condom conversation alongside whatever contraception you use.