Plan B works best within the first 24 hours after unprotected sex, when it prevents about 95% of pregnancies. Its effectiveness drops steadily after that, and while it can still reduce pregnancy risk up to 72 hours (three days), taking it sooner always gives you better odds. The pill itself clears from your body within a few days, and an unopened box stays viable in your medicine cabinet until the printed expiration date.
Most people asking “how long does Plan B last” want to know one of three things: how long they have to take it, how long it works inside the body, or how long it lasts on the shelf. Here’s all three.
How Long You Have to Take It
Plan B is most effective in the first 24 hours, preventing roughly 95% of pregnancies that would have otherwise occurred. After that, the numbers decline. By 48 to 72 hours, it still lowers your risk, but less dramatically. CDC guidelines from 2024 note that emergency contraceptive pills can reduce pregnancy risk through the fifth day (120 hours) after unprotected sex, but pregnancy rates are noticeably higher when taken after day three.
If you’re beyond the 72-hour mark, a different emergency contraceptive called ella (which requires a prescription) is more effective than Plan B in that three-to-five-day window. The two options perform similarly when taken within the first three days, but ella holds its effectiveness longer.
How It Works Inside Your Body
Plan B contains a large dose of a synthetic hormone that your body also uses in regular birth control pills. Its primary job is delaying ovulation. If your ovary hasn’t released an egg yet, the pill postpones that release long enough for sperm to die off (sperm typically survive three to five days in the reproductive tract). No egg, no fertilization, no pregnancy.
This is also why timing matters so much. If ovulation has already happened, Plan B has little to no effect. It does not interrupt an existing pregnancy.
In clinical studies, women who took Plan B around the time of ovulation experienced an average delay of about 17 days before ovulation actually occurred. That’s a significant pause, well beyond the lifespan of any waiting sperm.
How Long It Stays in Your System
The active ingredient in Plan B has a half-life of roughly 27 to 28 hours, according to FDA pharmacology data. That means about half the drug is eliminated from your body every 27 hours or so. After five to six half-lives, the drug is essentially gone. For most people, that works out to about six to seven days before it fully clears your system.
During that time, you may notice side effects like nausea, fatigue, headache, or breast tenderness. These typically fade within a day or two, well before the drug is completely eliminated.
Effects on Your Next Period
Plan B commonly shifts the timing of your next period. It may arrive earlier than expected, later than expected, or look different from your usual cycle. Heavier bleeding, lighter bleeding, and spotting are all normal responses. A period that’s a few days late after taking Plan B is not unusual and doesn’t necessarily signal pregnancy.
If your period is more than a week late, a pregnancy test is a reasonable next step.
Shelf Life of an Unopened Pill
An unopened Plan B box lasts until the expiration date printed on the packaging, which is typically a few years from the date of manufacture. Store it between 59 and 77 degrees Fahrenheit, in a cool and dry place away from direct sunlight. Extreme heat, cold, or sun exposure can degrade the active ingredient even if the pill looks and smells normal. A glove compartment in summer or a bathroom with frequent steam are not ideal storage spots.
If the expiration date has passed, the pill may still have some effect, but there’s no way to know how much potency it has lost. When pregnancy prevention is the goal, relying on an expired pill is a gamble.
Weight and Effectiveness
Plan B becomes significantly less effective at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or above experienced Plan B failure four times as often as those with a BMI under 25. Blood levels of the active ingredient were about 50% lower in the higher-BMI group after a standard dose, meaning the drug likely never reaches a high enough concentration to reliably block ovulation.
Doubling the dose does not solve this problem. A study from the same research group found that taking two Plan B pills did not meaningfully improve effectiveness for people with higher BMIs. For those in this weight range, ella or a copper IUD (which can be placed up to five days after unprotected sex) are more reliable options for emergency contraception.