Plan B starts working almost immediately after you swallow it, with the active ingredient reaching effective levels in your bloodstream within a few hours. But “working” in this case doesn’t mean what most people think. Plan B doesn’t terminate or block anything after the fact. It prevents pregnancy by delaying or stopping ovulation, so the speed that matters most isn’t how fast the pill dissolves but how quickly you take it after unprotected sex.
What Plan B Actually Does in Your Body
Plan B contains a high dose of a synthetic hormone that your body already produces naturally. Once absorbed, it interrupts the chain of hormonal signals that triggers ovulation, the release of an egg from the ovary. Specifically, it suppresses the surge of luteinizing hormone that causes a mature egg to rupture from its follicle. No egg released means no fertilization.
This is why timing is everything. Plan B is highly effective when taken during the first half of your cycle, before ovulation has occurred. FDA clinical data confirm that when taken before the hormonal surge, the drug reliably prevents the egg from being released. But once ovulation has already happened, Plan B has little to no effect on preventing pregnancy. It does not block fertilization after an egg is already in the fallopian tube, and it does not prevent a fertilized egg from implanting in the uterus.
How Quickly You Need to Take It
The FDA recommends taking Plan B as soon as possible within 72 hours (three days) of unprotected sex. Every hour you wait reduces how well it works. The numbers make this clear:
- Within 24 hours: about 94% effective at preventing pregnancy
- Within 72 hours: about 58% effective
That’s a dramatic drop. Taking Plan B on day one versus day three is the difference between a highly reliable method and one that fails nearly half the time. If you have it available, take it right away rather than waiting to see how you feel about it.
How Long It Stays in Your System
The active ingredient has an elimination half-life of roughly 24 to 28 hours, meaning half the dose clears your body in about a day. After two to three days, most of it is gone. Side effects, if you get them, tend to match this timeline. You might experience nausea, a headache, dizziness, breast tenderness, or light spotting. These are usually mild and short-lived, resolving within a day or two.
When Plan B Won’t Be Effective
Because Plan B works by delaying ovulation, it can only help if ovulation hasn’t happened yet. If you had unprotected sex on the day you ovulated or after, the pill is unlikely to prevent pregnancy. The problem is that most people don’t know exactly when they ovulate. Cycle-tracking apps give estimates, but they’re not precise enough to rely on for this kind of decision. When in doubt, taking Plan B is still reasonable since there’s no harm if ovulation has already passed.
Body weight also plays a role. Evidence suggests Plan B may be less effective in people who weigh more than 165 pounds, and it may not work at all above 176 pounds. Health Canada is currently the only regulator with an official recommendation about this, but the data are worth knowing. If you weigh more than 165 pounds, a prescription alternative that uses a different active ingredient remains effective up to 120 hours (five days) after sex and works better across a wider weight range. That option is about 98% effective within 24 hours and still roughly 85% effective at the five-day mark.
How to Know If It Worked
There’s no immediate sign that tells you Plan B was successful. You won’t feel anything different in the hours after taking it, aside from possible side effects. The real confirmation comes with your next period. Plan B can shift your cycle by a few days in either direction, so your period arriving a little early or a little late is normal and not a reason to worry.
If your period is more than five days late, take a pregnancy test. That’s the threshold recommended on the product labeling, and it gives enough time for a test to be accurate. A period that arrives on time, even if the flow is lighter or heavier than usual, generally means the pill did its job.
Plan B vs. the Prescription Alternative
Plan B is available over the counter with no age restriction, which makes it the most accessible option in an urgent situation. But it has two significant limitations: the 72-hour window and reduced effectiveness at higher body weights. The prescription alternative extends the effective window to 120 hours and maintains stronger efficacy regardless of when in your cycle you take it, because it can delay ovulation even after the hormonal surge has started, something Plan B cannot do.
If you’re within 24 hours of unprotected sex and weigh under 165 pounds, Plan B is a strong choice. If you’re closer to the 72-hour mark, weigh more than 165 pounds, or can get to a pharmacy or telehealth provider quickly, the prescription option offers meaningfully better protection.