Plan B is an emergency contraceptive pill that helps prevent pregnancy after unprotected sex or birth control failure. It contains a single dose of a synthetic hormone called levonorgestrel (1.5 mg) and works best when taken as soon as possible, within 72 hours. It is not an abortion pill. It prevents pregnancy from happening in the first place, primarily by delaying or blocking ovulation so that no egg is available to be fertilized.
How Plan B Works
Your body releases an egg from the ovary roughly once per menstrual cycle. Plan B delivers a concentrated burst of levonorgestrel, a synthetic version of progesterone, that signals the ovary to hold off on releasing that egg. If ovulation doesn’t happen, sperm have nothing to fertilize and pregnancy can’t begin.
This is the key distinction between Plan B and the abortion pill (a two-drug regimen of mifepristone and misoprostol used to end an existing pregnancy). Plan B cannot interrupt a pregnancy that has already been established. Once a fertilized egg has implanted in the uterus, the pill has no effect. The World Health Organization states plainly that emergency contraceptive pills do not induce an abortion and cannot harm a developing embryo.
How Effective It Is
Plan B prevents roughly 81 to 90 percent of expected pregnancies, depending on how quickly you take it. The single biggest factor in effectiveness is timing: the sooner you take it after unprotected sex, the better it works. The FDA recommends taking it as soon as possible within 72 hours (three days).
You can still take it between 73 and 120 hours (three to five days) after sex, but effectiveness drops noticeably in that window. Plan B is not meant to replace regular contraception. It’s a backup option for situations like a condom breaking, missing multiple birth control pills, or having unprotected sex.
Weight and Effectiveness
Plan B becomes significantly less reliable at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or above experienced pill failure four times as often as those with a BMI under 25. The reason is straightforward: blood levels of levonorgestrel were about 50 percent lower in people with a BMI of 30 after taking a standard dose, meaning the drug never reaches the concentration needed to reliably block ovulation. Doubling the dose did not solve this problem.
The general threshold cited in the literature is around 155 to 165 pounds, above which Plan B may not work well. If you weigh more than that, a prescription emergency contraceptive called ella (ulipristal acetate) is effective in people up to about 195 pounds and can be taken within five days of unprotected sex. A copper IUD, inserted by a healthcare provider within five days, is the most effective emergency contraceptive option regardless of weight.
Side Effects
Side effects are usually mild and short-lived, typically lasting only a few days. The most common ones include nausea, fatigue, headache, dizziness, breast tenderness, and stomach cramps. Some people experience light spotting between periods or a heavier flow during their next period.
Your next period may arrive a few days earlier or later than expected. If it’s more than a week late, a pregnancy test is a good idea, since no emergency contraceptive works 100 percent of the time. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose.
How to Get It
Plan B is available over the counter in the United States with no prescription, no ID, and no age requirement. You can buy it at pharmacies, drugstores, and online. Several generic versions exist under names like My Way, Take Action, and AfterPill, all containing the same 1.5 mg levonorgestrel dose. Generics are typically cheaper, sometimes significantly so, and work the same way.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down levonorgestrel, which can make Plan B less effective or even ineffective. The most notable category includes medications for epilepsy (such as carbamazepine, phenytoin, and barbiturates), tuberculosis drugs like rifampicin, and some HIV medications. The antiretroviral efavirenz, for example, cuts levonorgestrel blood levels by about 50 percent.
St. John’s wort, an herbal supplement commonly taken for mood, also reduces levonorgestrel levels. This enzyme-boosting effect can persist for up to four weeks after stopping the interfering medication, so even recently discontinued drugs can be relevant. If you take any of these, ella or a copper IUD are better emergency options.
Plan B vs. Ella
Both Plan B and ella are emergency contraceptive pills that work by delaying ovulation, but they differ in important ways. Plan B is available without a prescription and works best within 72 hours. Ella requires a prescription, remains effective for up to five days, and works better for people at higher body weights. Both lose effectiveness the longer you wait, so speed still matters with either option. Neither one ends an existing pregnancy.