Plan B is most effective when taken within 72 hours (3 days) after unprotected sex, and it works best the sooner you take it. The pill is not an abortion pill; it works by delaying ovulation so sperm cannot meet an egg. Taking it within 24 hours gives you the highest chance of preventing pregnancy, but it still offers meaningful protection through day 3.
How Plan B Works in the Body
Plan B One-Step contains a hormone called levonorgestrel. This is a synthetic form of progesterone, a natural hormone involved in the menstrual cycle. The pill works primarily by stopping or delaying the ovary from releasing an egg.
If ovulation has already happened, Plan B generally will not work. The sperm may have already fertilized the egg. For this reason, timing matters more than almost anything else.
Plan B does not end a pregnancy. It does not harm an existing pregnancy. It does not affect a fertilized egg that has already implanted in the uterus. These are common misunderstandings that lead people to delay taking the pill or avoid it altogether.
When Is Plan B Most Effective
Plan B is most effective when taken within the first 24 hours after unprotected sex. Studies consistently show that the sooner you take the pill, the higher the chance it will prevent pregnancy.
The pill is labeled for use up to 72 hours after unprotected sex. Some research suggests it may have some effect up to 120 hours (5 days), but the evidence for effectiveness beyond 72 hours is less consistent. Do not wait for this window to close. If you need Plan B, take it as soon as you can.
Effectiveness also depends on where you are in your menstrual cycle. If you are close to ovulation, the pill has less time to delay the egg’s release. If you are days away from ovulation, the pill has a better chance of working.
Body weight also plays a role. Studies have found that levonorgestrel-based emergency contraception like Plan B may be less effective in people who weigh more than 165 pounds (75 kg). For people over 175 pounds (80 kg), the failure rate is noticeably higher. This does not mean the pill does nothing — it means it may not work as well, and a different option may be better.
How Effective Is Plan B by Hour
Effectiveness declines steadily over the 72-hour window. The following breakdown reflects what research has shown about levonorgestrel emergency contraception:
- Within 24 hours: Highest effectiveness. Most studies show the risk of pregnancy is reduced by about 95% when taken this quickly.
- 24 to 48 hours: Effectiveness drops slightly. The risk of pregnancy is reduced by roughly 85%.
- 48 to 72 hours: Effectiveness continues to decline. The risk of pregnancy is reduced by about 58%.
These percentages are based on clinical studies of levonorgestrel emergency contraception. They represent the reduction in pregnancy risk compared to using no contraception. No method is 100% effective, and individual results vary.
If you are already ovulating or ovulated within the past day, the numbers above may overstate your protection. The pill cannot reverse ovulation once it has occurred.
Weight Limits and Alternatives
If you weigh more than 165 pounds, Plan B may not be your best option. The evidence is strong enough that many clinicians recommend a different emergency contraceptive for people above this weight range.
Two alternatives exist:
Ulipristal acetate (ella): This is a prescription emergency contraceptive that works differently from Plan B. It blocks progesterone receptors and can delay ovulation even when ovulation is close. Studies indicate it is more effective than levonorgestrel, especially later in the 72-hour window. It also appears to work regardless of body weight, though some research suggests reduced effectiveness in people over 195 pounds (88 kg).
Copper IUD (Paragard): The copper intrauterine device is the most effective form of emergency contraception. When inserted within 5 days of unprotected sex, it prevents pregnancy in more than 99% of cases. It works regardless of weight. It also provides ongoing contraception for up to 10 years.
If you weigh over 165 pounds and want the most reliable emergency contraception, ask a healthcare provider about ella or a copper IUD. These options require a prescription or a clinic visit, but they are significantly more effective for people in higher weight ranges.
How to Take Plan B Correctly
Plan B One-Step is a single pill. Take it as soon as possible after unprotected sex. Do not split the dose. Do not take two pills unless the package instructions specifically say to do so — some generic versions contain two pills meant to be taken together or as directed.
You do not need to eat before taking Plan B. Food does not affect how well the pill works. If you vomit within 2 hours of taking the pill, contact a healthcare provider or pharmacist. You may need to take another dose.
Plan B is available over the counter at most pharmacies. You do not need a prescription and you do not need to show identification. It is typically located in the family planning aisle, not behind the counter.
Side Effects and What to Expect
Plan B can cause temporary side effects. These are not dangerous and usually resolve within a day or two:
- Nausea — the most common side effect
- Abdominal pain or cramping
- Fatigue
- Headache
- Breast tenderness
- Dizziness
Your next period may come earlier or later than expected. It may be heavier or lighter than usual. This is a normal response to the hormone surge.
If your period is more than 7 days late, take a pregnancy test. If your period is unusually light or you have other symptoms of pregnancy, test even if your period is not late. Plan B is not 100% effective, and a late period is the most common first sign that it did not work.
Does Plan B Affect Your Future Fertility
No. Taking Plan B does not affect your ability to get pregnant in the future. The hormone leaves your system within a few days, and your next menstrual cycle returns to normal.
Some people worry that frequent use of emergency contraception is harmful. It is not. You can take Plan B more than once in a single menstrual cycle. However, it is not designed for regular use. It is less effective than ongoing birth control methods like the pill, patch, ring, implant, or IUD. If you find yourself needing emergency contraception often, talk to a healthcare provider about a more reliable daily or long-acting method.
When to See a Healthcare Provider
See a provider if you experience severe abdominal pain that does not go away, especially on one side. This could be a sign of an ectopic pregnancy — a pregnancy that implants outside the uterus. Ectopic pregnancy is a medical emergency.
Also see a provider if you have heavy bleeding, fever, or signs of infection after taking Plan B. These symptoms are not typical side effects of the pill and warrant evaluation.
If you took Plan B and your period is more than a week late, a pregnancy test is the next step. If the test is positive, make an appointment with a provider to discuss your options and confirm the pregnancy is in the right place.
Frequently Asked Questions
Can I take Plan B more than once in a cycle?
Yes, you can take Plan B multiple times in one menstrual cycle. It will not harm you, but it is less effective than regular birth control, so talk to a provider about a more reliable method.
Does Plan B work if I already ovulated?
No. Plan B works by delaying ovulation, so it cannot prevent pregnancy if ovulation has already occurred. This is why taking it as soon as possible matters.
How late can I take Plan B?
Plan B is labeled for use up to 72 hours after unprotected sex. Some research suggests limited effectiveness up to 120 hours, but do not wait — take it immediately.
Is Plan B the same as the abortion pill?
No. Plan B prevents ovulation and does not end an existing pregnancy. The abortion pill, mifepristone, is a different medication used to end a pregnancy that has already implanted.
