How Effective Is Plan B Within 24 Hours?

If you take Plan B (levonorgestrel) within 24 hours of unprotected sex, it is highly effective at preventing pregnancy. Research consistently shows that taking this emergency contraceptive within the first 24 hours works in about 95% of cases when used correctly. The key point is simple: the sooner you take it, the better it works, and taking it within 24 hours puts you in the window where it is most reliable.

How Effective Is Plan B Within 24 Hours

Plan B is one of the most studied emergency contraceptives available. When taken within 24 hours, studies have found it prevents pregnancy in roughly 95 out of 100 cases. That number drops slightly as time passes, falling to about 85% effectiveness between 25 and 48 hours, and continuing to decline until the 72-hour mark, which is the outer limit for this medication.

These figures come from clinical research on levonorgestrel-based emergency contraception. The medication works by delaying ovulation. If ovulation has already happened, Plan B will not work. This is why timing matters so much — you need to take it before your ovary releases an egg.

Effectiveness also depends on your weight. Some studies indicate that Plan B may be less effective in women who weigh more than 165 pounds (75 kg). For women over 175 pounds (80 kg), alternative options like ulipristal acetate (ella) or a copper IUD may be more reliable. This is not a reason to skip Plan B if it is your only option — it still offers meaningful protection — but it is important information for making a fully informed choice.

How Does Plan B Work in the Body

Plan B contains a synthetic hormone called levonorgestrel. This is a progestin, which is a laboratory-made version of the natural hormone progesterone. When you take the pill, it delivers a high dose of this hormone to your system.

The main job of this hormone surge is to stop or delay ovulation. Normally, a surge of luteinizing hormone (LH) triggers the ovary to release an egg. The high dose of levonorgestrel interferes with this process. If the egg never gets released, there is nothing for sperm to fertilize, and pregnancy cannot occur.

There is an important detail here that often gets confused. Plan B does not end an existing pregnancy. It does not work like mifepristone, the abortion pill. If a fertilized egg has already implanted in the uterus, Plan B will not disrupt that pregnancy. This is why it is classified as a contraceptive, not an abortifacient.

Some older descriptions of emergency contraception mentioned that levonorgestrel might also thicken cervical mucus or affect the uterine lining. Current evidence suggests these effects are minor or may not occur at all. The primary mechanism is ovulation delay, and that is what researchers focus on today.

How to Take Plan B Correctly

Plan B is sold as a single pill that you take all at once. The standard dose is 1.5 mg of levonorgestrel. Older versions required two doses taken 12 hours apart, but the single-dose formulation is now the standard in the United States.

You can take Plan B at any point during your menstrual cycle. The medication does not require you to be fasting, and it does not interact with most common foods. Take it with water, just like any other pill.

Vomiting is the main concern after taking Plan B. If you vomit within two hours of taking the pill, the dose may not have been fully absorbed. Contact a pharmacist or clinician for guidance. They may recommend taking a second dose or using an alternative method.

Plan B is available over the counter without a prescription or age restriction in the United States. You can find it in the family planning aisle of most pharmacies. It costs roughly $40 to $50, though generic versions of levonorgestrel are often less expensive and work identically.

What Happens After You Take Plan B

Most women will have some spotting or bleeding in the days after taking Plan B. This is a normal response to the sudden hormone change. Your next period may also come earlier or later than expected. It may be lighter or heavier than usual.

If your period is more than seven days late, take a pregnancy test. This is the standard medical advice because a delayed period can occasionally mean the contraception did not work. A home pregnancy test is reliable at this point.

Some women experience nausea, headache, breast tenderness, or fatigue after taking Plan B. These side effects usually resolve within a day or two. Nausea is the most common complaint, affecting roughly one in five women who take the medication. If you have a history of nausea with hormonal medications, you may want to ask a pharmacist about anti-nausea options.

Plan B is not meant for regular use. It contains a much higher hormone dose than daily birth control pills. Using it repeatedly can disrupt your cycle and cause unpredictable bleeding. If you find yourself needing emergency contraception often, talk to a clinician about a more reliable long-term method.

When Plan B Fails: Understanding the Limits

No emergency contraceptive is 100% effective. Even when taken perfectly within 24 hours, about 5% of women will still become pregnant. Understanding why this happens requires looking at the timing of your cycle.

Plan B only works if it can delay ovulation. If you are already ovulating when you take the pill, it cannot stop the process. The egg is already on its way, and sperm can survive in the female reproductive tract for up to five days. This means intercourse that happened before ovulation can still result in pregnancy if the egg is released after the sperm has arrived.

This is not a failure of the medication. It is a biological limitation. No pill can reverse ovulation that has already begun.

Certain medications can also reduce the effectiveness of Plan B. Drugs that affect liver enzymes, such as some anti-seizure medications, certain antibiotics, and the herb St. John’s Wort, can speed up how quickly your body breaks down levonorgestrel. If you take any of these regularly, ask a pharmacist whether Plan B is the right choice for you.

Alternatives to Plan B That May Work Better

If you are within five days (120 hours) of unprotected sex, you have several options beyond Plan B. Some of these are actually more effective, depending on your circumstances.

Ulipristal acetate, sold under the brand name ella, is a prescription emergency contraceptive. It works by blocking progesterone receptors in the body, which delays ovulation more effectively than levonorgestrel. Some studies suggest ella remains highly effective for the full five days, while Plan B loses potency after the first 24 hours. Ella requires a prescription from a clinician.

The copper IUD (ParaGard) is the most effective form of emergency contraception. When inserted within five days of unprotected sex, it prevents pregnancy in more than 99% of cases. It works by creating an environment in the uterus that is toxic to sperm and eggs. As a bonus, it provides ongoing contraception for up to 10 years. Not all clinics carry it for emergency insertion, so call ahead.

The hormonal IUD (Mirena, Liletta) has also been studied for emergency use, but the copper IUD has the strongest evidence and is the standard recommendation. Some recent research suggests hormonal IUDs may also work, but guidelines have not universally adopted this practice.

Common Myths About Plan B

One widespread myth is that Plan B causes an abortion. It does not. Medical organizations, including the American College of Obstetricians and Gynecologists, classify it as a contraceptive because it prevents fertilization. It cannot terminate an established pregnancy.

Another myth is that taking multiple doses of Plan B increases effectiveness. It does not. Taking two pills at once only increases side effects like nausea and does not improve your chances of preventing pregnancy. The 1.5 mg dose is already the maximum studied dose.

Some people believe Plan B works for several days after sex, like a “morning after” pill that can be taken anytime in the following week. This is incorrect. Plan B is only approved for use within 72 hours, and its effectiveness declines significantly after the first 24 hours. If you are past 72 hours, ask a clinician about ella or a copper IUD.

Finally, Plan B does not protect against sexually transmitted infections. It only prevents pregnancy. If you have concerns about STI exposure, talk to a clinician about testing and prevention.