How Effective Is Plan B? Failure Rate, Timing, and Efficacy

Plan B One‑Step prevents approximately 97–98% of expected pregnancies when taken within the recommended 72‑hour window, according to clinical data reviewed by the U.S. Food and Drug Administration (FDA). This article breaks down effectiveness rates by time elapsed after unprotected sex, explains the failure rate in real numbers, and identifies which medications or factors can reduce how well the morning‑after pill works. All claims come from peer‑reviewed studies, government health agencies, and established medical information platforms.

Last checked: 2026-05-24

Best time to take: Within 72 hours of unprotected sex; optimal within 24 hours · Peak effectiveness: 98% if taken within 24 hours · Declining effectiveness: 85% at 72–120 hours · Overall effectiveness range: 89% to 98% depending on timing

How effective is Plan B within 24 hours?

The sooner Plan B One‑Step is taken after unprotected sex, the better it works, according to the FDA. When swallowed within the first 24 hours, the single‑dose 1.5 mg levonorgestrel pill prevents approximately 97–98% of expected pregnancies, based on clinical data reviewed by Rochester Regional Health. Wake Forest University’s Deacon Health service reports that taken in the first 24 hours, Plan B One‑Step can prevent about 95% of expected pregnancies, with effectiveness declining as the interval increases.

The drug works primarily by stopping or delaying ovulation—the release of an egg from the ovary—before fertilization can occur. The FDA states plainly that evidence does not support an effect on implantation or disruption of an existing pregnancy. This means Plan B cannot terminate an established pregnancy, which is a common point of confusion.

Effectiveness breakdown by hour

Clinical research published in the Open Access Journal of Contraception via PubMed Central shows that levonorgestrel emergency contraceptive pills produce pregnancy rates of approximately 1–3% among users—substantially lower than the expected 5–8% without treatment. The range reflects variable factors such as where a woman is in her menstrual cycle and whether ovulation has already occurred.

Comparison with other EC methods

Plan B’s 24‑hour effectiveness rate of roughly 97–98% compares favorably with other oral emergency contraceptive options. According to peer‑reviewed research via PubMed Central, ulipristal acetate (ella), the most effective prescription emergency contraceptive pill, shows failure rates of 0.9–2.1% in clinical trials. A copper IUD inserted within 5 days of unprotected intercourse is the most effective form of emergency contraception overall, preventing more than 99% of expected pregnancies.

What is the failure rate of Plan B?

No emergency contraceptive is 100% effective, and Plan B is no exception. When taken within 72 hours, the pill is roughly 89% effective at preventing pregnancy, according to GoodRx Health’s summary of clinical trial data. The official Plan B One‑Step website states that when used as directed within the 72‑hour window, about 7 out of every 8 women who could have gotten pregnant did not become pregnant.

Pregnancy rates with and without Plan B

The baseline risk without any intervention is well established. A review hosted by the National Institutes of Health (NIH) via PubMed Central found that about 8 in 100 women are expected to become pregnant after a single act of unprotected sex. When Plan B is taken as directed, that number drops to approximately 1–3%, according to clinical trial data reviewed in the Open Access Journal of Contraception.

Deacon Health at Wake Forest University puts this into concrete terms: after a single act of unprotected intercourse, Plan B One‑Step reduces the risk of pregnancy by 89% and lowers average pregnancy risk from about 8% to about 1%. This means that for every 100 women who take Plan B after unprotected sex, roughly 1 to 3 may still become pregnant.

Factors affecting failure rate

Several factors can influence whether Plan B fails to prevent pregnancy. According to Clue’s health platform, one‑dose emergency contraception pills are estimated to prevent pregnancy in roughly 50–100% of cases, with the wide range reflecting uncertainty about ovulation timing and baseline pregnancy risk. If a woman has already ovulated before taking the medication, the chance of pregnancy is substantially higher.

How long does Plan B remain effective?

The FDA’s drug safety information for Plan B One‑Step states that the most important factor affecting how well levonorgestrel emergency contraception works is how quickly it is taken after unprotected sex. Plan B should be taken as soon as possible within 72 hours (3 days) to help prevent pregnancy before it starts.

Best window: within 72 hours

The standard clinical window for levonorgestrel emergency contraception is 72 hours, with effectiveness declining notably after 24 hours. According to the official Plan B One‑Step FAQ, the sooner the pill is taken after unprotected sex, the better it works, and it must be taken within 72 hours to help prevent pregnancy. The 7‑out‑of‑8 statistic applies specifically to women who take the medication within this 3‑day window.

Efficacy beyond 72 hours

While the FDA recommends taking Plan B within 72 hours, clinical evidence shows it can still reduce pregnancy risk when taken up to 120 hours (5 days) after unprotected sex, albeit with reduced efficacy. Rochester Regional Health reports effectiveness of approximately 85% at the 72–120 hour mark. The Clue health article confirms that levonorgestrel emergency contraception should ideally be taken within 3 days but can be taken up to 5 days after sex, though effectiveness decreases in that extended window.

What this means: If more than 72 hours have passed since unprotected intercourse, Plan B is still better than doing nothing, but women should be aware the protection is lower. A copper IUD remains the most effective emergency contraceptive option up to 5 days after intercourse, with greater than 99% effectiveness.

What cancels a Plan B out?

Several conditions and substances can reduce Plan B’s effectiveness or render it ineffective. Understanding these factors is essential for anyone relying on the morning‑after pill after unprotected sex or contraceptive failure.

Medication interactions

Certain prescription and over‑the‑counter medications can decrease how well Plan B works. Healthgrades, citing Planned Parenthood, warns that St. John’s wort, some HIV antivirals, barbiturates, anticonvulsants, and other seizure medications can reduce the effectiveness of Plan B or its generic equivalent Take Action. These drugs accelerate the metabolism of levonorgestrel, lowering blood concentrations of the active ingredient.

Vomiting within 2–3 hours

If nausea causes vomiting within 2–3 hours of taking Plan B, the dose may not be fully absorbed. Women who vomit shortly after taking the pill should consult a pharmacist or healthcare provider about whether a repeat dose is needed. According to GoodRx Health, common side effects include nausea, fatigue, headache, dizziness, breast tenderness, and temporary changes in menstrual bleeding or timing—any of which can accompany the gastrointestinal upset that some users experience.

Weight considerations

Research suggests that body weight may influence how effective levonorgestrel emergency contraception is. Kaiser Family Foundation (KFF) notes that the progestin‑only pill regimen for emergency contraception typically has a pregnancy risk reduction of about 65% compared with no emergency contraception, with reported failure rates in clinical trials around 2%. Some studies indicate reduced efficacy in women over 165 pounds, though data remains variable.

Already pregnant

According to the FDA, Plan B One‑Step will not work if a person is already pregnant and does not terminate or harm an existing pregnancy. The medication’s mechanism is ovulation inhibition, meaning it cannot prevent pregnancy after implantation has occurred. Women who suspect they may already be pregnant should take a pregnancy test before taking Plan B.