You have up to 120 hours (5 days) to take Plan B after unprotected sex, but it works best when taken within the first 72 hours (3 days). The sooner you take it, the more effective it is. Plan B is a type of emergency contraception that prevents pregnancy by delaying or stopping ovulation. It is not an abortion pill and will not work if a fertilized egg has already implanted.
How Long Do U Have To Take Plan B for It to Work?
Plan B is FDA-approved for use up to 72 hours after unprotected sex. However, some studies show it can still reduce pregnancy risk if taken up to 120 hours (5 days) after. Effectiveness drops the longer you wait. Within 24 hours, Plan B prevents pregnancy about 95% of the time. By 72 hours, that drops to around 89%. By 120 hours, effectiveness is much lower, though some protection remains.
Research published in the journal Contraceptive found that women who took Plan B within 24 hours had a pregnancy rate of 0.4%. Those who waited 72 hours had a rate of 2.7%. The key point is that every hour matters. If you are close to 72 hours, take it immediately. Do not wait for the 5-day mark if you can take it sooner.
One common misunderstanding is that Plan B works exactly the same at 5 days as it does at 1 day. This is false. The pill’s ability to stop ovulation weakens over time. If you are already ovulating or have ovulated recently, Plan B may not work at all.
Does Timing Depend on Your Menstrual Cycle?
Yes. Plan B works by delaying ovulation. If you have not ovulated yet, it can prevent the egg from being released. If you have already ovulated, Plan B cannot stop pregnancy because the egg is already available for fertilization. This is why timing relative to your cycle matters just as much as timing after sex.
Women with regular 28-day cycles typically ovulate around day 14. If you have unprotected sex on day 10, Plan B has a good chance of working because ovulation has not happened yet. If you have sex on day 16, ovulation may have already occurred, and Plan B will not help. The contraceptive effectiveness of Plan B depends entirely on where you are in your cycle.
If your cycles are irregular, it is harder to predict ovulation. In that case, taking Plan B as soon as possible after unprotected sex is your best option. The CDC recommends that anyone who has had unprotected sex and does not want to become pregnant should take emergency contraception without delay, regardless of cycle day.
What Factors Affect How Well Plan B Works?
Body weight is one factor. Research shows that Plan B may be less effective in women with a BMI over 25 and significantly less effective in women with a BMI over 30. For women with a BMI over 25, the pregnancy rate after taking Plan B was about 3 times higher than for women with a lower BMI. For women with a BMI over 30, some studies suggest Plan B may not work at all.
Other medications can also reduce effectiveness. Drugs that affect liver enzymes, such as certain seizure medications, antibiotics like rifampin, and the herbal supplement St. John’s Wort, can make Plan B less reliable. If you take any of these, talk to a pharmacist or doctor about alternatives.
Timing of sex relative to ovulation is the biggest factor. If you have sex right before or during ovulation, no emergency contraceptive is guaranteed. The only way to know for sure is to take Plan B as quickly as possible and watch for your next period.
What Are the Side Effects of Plan B?
Plan B can cause nausea, vomiting, fatigue, headache, breast tenderness, and irregular bleeding. These side effects are usually mild and last 1-2 days. Some women experience spotting or changes in their next period. Your period may come earlier or later than expected, often by a week or less.
If you vomit within 2 hours of taking Plan B, the dose may not be fully absorbed. In that case, contact a healthcare provider. They may recommend taking another dose or using a different type of emergency contraception, like the copper IUD, which is the most effective option and works for up to 5 days after sex regardless of body weight.
Side effects are not a sign that Plan B worked or did not work. Some women have no side effects and still do not get pregnant. Others have side effects and do get pregnant. The only way to confirm is to take a pregnancy test if your period is more than 7 days late.
How Do Other Emergency Contraceptives Compare?
There are three main types of emergency contraception: Plan B (levonorgestrel), ella (ulipristal acetate), and the copper IUD (Paragard). They work differently and have different windows of effectiveness.
| Method | Time Window | Effectiveness | Weight Limit |
|---|---|---|---|
| Plan B (levonorgestrel) | Up to 72 hours (some effect up to 120 hours) | 89-95% within 72 hours | Less effective if BMI over 25 |
| ella (ulipristal acetate) | Up to 120 hours (5 days) | 95% or higher throughout 5 days | Effective regardless of BMI |
| Copper IUD | Up to 120 hours (5 days) | Over 99% | No weight limit |
ella is a prescription medication that works closer to ovulation than Plan B. Studies show ella remains effective even when taken near the time of ovulation. Plan B loses effectiveness if taken too close to ovulation. The copper IUD is the most effective option and also provides long-term birth control once inserted.
According to the American College of Obstetricians and Gynecologists, ella and the copper IUD are better choices if you have a BMI over 25 or if more than 72 hours have passed since unprotected sex. Plan B is still a good option if taken quickly and your BMI is under 25.
Common Misconceptions About Plan B
One myth is that Plan B causes abortion. It does not. Plan B prevents pregnancy by stopping ovulation. It has no effect on a fertilized egg that has already implanted in the uterus. It is not the same as the abortion pill (mifepristone). This distinction is important and supported by the FDA.
Another myth is that taking Plan B multiple times in one cycle is dangerous. It is not. You can take Plan B more than once in the same menstrual cycle if needed. However, it is not as effective as regular birth control, and repeated use can cause irregular bleeding. It should not be used as your main form of contraception.
Some people believe Plan B causes infertility. There is no evidence for this. Research has not found any link between emergency contraception and long-term fertility issues. Plan B does not affect your ability to get pregnant in future cycles.
Finally, many think Plan B works immediately after sex. This is misleading. Plan B works only if you have not already ovulated. If you take it after ovulation has started, it will not prevent pregnancy. This is why timing matters so much.
What to Do If Plan B Fails
If your period is more than 7 days late after taking Plan B, take a pregnancy test. Home pregnancy tests are reliable from the first day of a missed period. If the test is positive, contact a healthcare provider to discuss options. These include continuing the pregnancy, abortion, or adoption depending on your state’s laws.
If your period is on time, you are almost certainly not pregnant. Plan B can sometimes cause spotting that looks like a light period. If you are unsure, wait a few days and test again. False negatives are rare but possible if you test too early.
If you have unprotected sex again after taking Plan B, you need another dose. Plan B does not provide ongoing protection. It only works for the one instance of sex before taking it. Use condoms or another form of birth control until your next period to avoid needing emergency contraception again.
When to See a Doctor
See a doctor if you vomit within 2 hours of taking Plan B. They can advise whether you need another dose. Also see a doctor if you experience severe abdominal pain, heavy bleeding, or fainting after taking Plan B. These could be signs of an ectopic pregnancy, which is rare but serious.
If you have a BMI over 30 or take medications that interact with Plan B, talk to a doctor about more effective options like ella or the copper IUD. These are available by prescription and may be covered by insurance. Planned Parenthood and many clinics offer same-day appointments for emergency contraception.
If you need emergency contraception regularly, consider a more reliable long-term method. Birth control pills, implants, IUDs, and shots are all more effective than Plan B at preventing pregnancy. The copper IUD, for example, is over 99% effective for up to 12 years and can also be used as emergency contraception.
Frequently Asked Questions
Can I take Plan B after 5 days?
Plan B is not recommended after 5 days because effectiveness drops significantly. The copper IUD or ella are better options at that point.
Does Plan B work if I am already ovulating?
No. Plan B only prevents pregnancy by stopping ovulation. If ovulation has already occurred, it will not work.
Can Plan B mess up my period permanently?
No. Plan B can cause one cycle to be irregular, but it does not cause permanent changes to your menstrual cycle.
Is Plan B the same as the abortion pill?
No. Plan B prevents pregnancy before implantation. The abortion pill (mifepristone) ends a pregnancy after implantation. They are completely different medications.
