Plan B reaches its peak concentration in your bloodstream within about 1 to 2 hours of swallowing the pill. But “working” in the practical sense means something different: the hormone needs to intervene before ovulation happens, and whether it succeeds depends almost entirely on how soon you take it after unprotected sex. The sooner you take it, the more likely it is to prevent pregnancy.
What Happens Inside Your Body
Plan B delivers a large dose of a synthetic hormone that your body absorbs quickly. In clinical studies of 30 women, blood levels peaked at an average of 1.67 hours after taking the pill, with a range of 1 to 4 hours. Once absorbed, the hormone’s primary job is to delay or prevent your ovary from releasing an egg. If there’s no egg available, sperm can’t fertilize anything, and pregnancy doesn’t occur.
This is why timing matters so much. Plan B is not interrupting a pregnancy that has already started. It’s racing to block ovulation before sperm (which can survive in the reproductive tract for up to five days) reach a newly released egg. If ovulation has already happened, Plan B has little to no effect.
How Timing Affects Effectiveness
Plan B’s ability to prevent pregnancy drops sharply with every day you wait:
- Within 24 hours: Around 94% effective
- Within 72 hours (3 days): About 58% effective
The CDC recommends taking emergency contraception as soon as possible, within 5 days (120 hours) of unprotected sex. However, between 72 and 120 hours, Plan B’s effectiveness is considerably lower. If you’re in that later window, a different emergency contraceptive pill containing ulipristal acetate is more effective during those final two days. A copper IUD, inserted by a provider within five days, is the most effective emergency option at any point in that window.
Body Weight Can Reduce Effectiveness
One factor many people don’t realize: Plan B becomes significantly less effective at higher body weights. Research shows that people with a BMI of 30 or above had roughly four times the risk of pregnancy compared to those with a BMI under 25. The decline in effectiveness appears tied to absolute weight rather than BMI alone. Levonorgestrel-based emergency contraception hits a ceiling of efficacy around 70 kg (about 154 pounds) and shows essentially no efficacy for people weighing 80 kg (about 176 pounds) or more.
If you’re above those thresholds, ulipristal acetate or a copper IUD are better options. This isn’t about body shaming; it’s straightforward pharmacology. A fixed dose of any drug gets diluted in a larger body, and the hormone may not reach the concentration needed to block ovulation.
What Can Interfere With Absorption
Vomiting is the most common concern. If you throw up within 2 to 3 hours of taking Plan B, the pill may not have been fully absorbed, and you need to take another dose. If nausea is a concern, taking the pill with a small amount of food can help, though it’s more important to take it quickly than to wait for ideal conditions.
Certain medications also speed up how fast your liver breaks down the hormone, reducing its effectiveness. These include some drugs used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis medications like rifampicin, several HIV treatments including efavirenz (which cuts levonorgestrel blood levels by about 50%), and the herbal supplement St. John’s wort. Even after you stop taking one of these medications, the effect on your liver enzymes can persist for up to four weeks.
Side Effects and What to Expect Afterward
Most side effects are mild and short-lived. Nausea is the most commonly reported one, and some people experience headaches, fatigue, breast tenderness, or light spotting. These typically resolve within a day or two. Plan B causes less nausea than older combined-hormone emergency contraceptive regimens.
Your next period may come earlier or later than expected. A shift of a few days to a week in either direction is normal. If your period is more than three weeks late, take a pregnancy test. That delay doesn’t necessarily mean Plan B failed, but it’s worth confirming.
What to Do After Taking It
Plan B is a one-time intervention, not ongoing protection. You can start or resume regular birth control (the pill, patch, ring, or shot) immediately after taking it, but you’ll need to use condoms or abstain for seven days while that method takes effect. Plan B does not protect against any future unprotected sex, even if it happens later the same day.