Plan B (Levonorgestrel): Facts About Emergency Contraception

Plan B works best when taken quickly after unprotected sex — here is what it does, and what it does not.

Emergency contraception is meant for the moments when routine methods fail or were not used at all: a torn condom, a missed pill, or an unplanned situation. Plan B, whose active ingredient is levonorgestrel, is one of the most studied options for this scenario. Understanding what it does — and what it does not — helps people make calm, informed decisions when time matters.

How Plan B works

Levonorgestrel is a synthetic hormone similar to progestin. It works mainly by delaying or preventing ovulation, so the egg is not released and fertilization does not occur. If ovulation has already happened, its effectiveness drops. Plan B does not terminate an established pregnancy.

Timing is everything

Effectiveness declines rapidly after unprotected intercourse. Taken within 24 hours, Plan B reduces pregnancy risk substantially. It can still help up to 72 hours later, but the sooner it is used, the better it works. If more than three days have passed, other emergency contraceptive options may be more appropriate — a healthcare professional can advise.

What to expect after taking it

Common short-term effects include nausea, headache, breast tenderness, and slight changes in the next period. Some users bleed a few days earlier or later. If the next period is more than a week late, take a pregnancy test.

Available packages

The Plan B page lists several pack sizes so people can be prepared before they need it, not after:

1.5 mg × 8 tablets — a small backup for personal use.
1.5 mg × 12 or 18 tablets — for households or small clinics.
1.5 mg × 24 or 30 tablets — larger supply for community health services and travel kits.

Having a dose at home can turn a stressful night into a calm decision. Plan B is not intended as a regular contraceptive; frequent use is less effective and less comfortable than a planned method.

What comes next

Emergency contraception is a chance to think about a longer-term plan: a daily pill, an implant, an IUD or another method that matches your life. Talk to a healthcare provider and, in the meantime, keep a backup pack from the Plan B page.