Plan B is a form of emergency contraception that works best the sooner you take it. It is not an abortion pill, and it will not end a pregnancy that has already implanted. The key question is timing: Plan B is most effective within the first 72 hours (3 days) after unprotected sex, and its effectiveness drops steadily as time passes.
When Is It Too Late For Plan B
Plan B is officially approved for use up to 72 hours after unprotected sex. Taking it within the first 24 hours gives the highest chance of preventing pregnancy. After 72 hours, the pill becomes significantly less effective, and other options may work better.
Some research suggests Plan B may still have some effect up to 120 hours (5 days) after sex, but the evidence here is less consistent. The most reliable guidance remains: take it as soon as possible, ideally within 24 hours and no later than 72 hours. Waiting until day 4 or 5 means you are relying on weaker evidence and a lower chance of success.
If you are already past 72 hours, do not assume nothing can be done. A copper IUD can be inserted up to 5 days after unprotected sex and is actually more effective than Plan B at any time point. This is a real option worth discussing with a clinician immediately.
How Plan B Works in the Body
Plan B contains levonorgestrel, a synthetic hormone similar to progesterone. Its main job is to delay or stop the release of an egg from the ovary. If no egg is released, sperm has nothing to fertilize, and pregnancy cannot begin.
The pill does not work after fertilization has already happened. It does not disrupt an implanted embryo, and it does not affect an established pregnancy. This is why timing matters so much — the pill only has a window of opportunity before ovulation occurs.
Ovulation timing varies from person to person. If you have already ovulated when you take Plan B, the pill likely will not work. This is one reason no emergency contraceptive is 100% effective. The closer you are to ovulation when you have sex, the higher your risk of pregnancy, and the less reliable Plan B becomes.
Does Weight Affect How Well Plan B Works?
Yes, body weight appears to influence Plan B’s effectiveness, though the research is not fully settled. Some studies have found that levonorgestrel-based emergency contraception like Plan B is less effective in people who weigh more than about 165 pounds (75 kg).
For people over 175 pounds (about 80 kg), the failure rate appears higher. This does not mean the pill does nothing — it means the odds are worse. A different emergency contraceptive called ella (ulipristal acetate) may be more effective for people in this weight range, though it also has its own weight limitations.
If you weigh more than 165 pounds, talk to a pharmacist or clinician about alternatives. The copper IUD is not affected by weight at all and remains highly effective regardless of body size.
Plan B vs. Other Emergency Contraception Options
Plan B is not the only emergency contraception available, and it is not always the best choice. Understanding your options matters because timing and individual factors change which approach is most appropriate.
| Method | Time Window | Prescription Needed? | Weight Limitations |
|---|---|---|---|
| Plan B (levonorgestrel) | Up to 72 hours | No — available over the counter | May be less effective above 165 lbs |
| ella (ulipristal acetate) | Up to 120 hours | Yes | May be less effective above 195 lbs |
| Copper IUD | Up to 120 hours | Yes — insertion by clinician | None |
ella is a prescription pill that blocks progesterone from working, which delays ovulation differently than Plan B. Some research suggests ella remains more effective than Plan B in the later part of the 5-day window. However, ella requires a doctor’s visit or a telehealth appointment, which may not be possible for everyone on short notice.
The copper IUD is the most effective emergency contraception available. It works by creating an environment in the uterus that is hostile to sperm and eggs. It is also a long-term birth control method once inserted, lasting up to 10 years.
Common Myths About Plan B That Need Clearing Up
There is a lot of misinformation about emergency contraception. Some myths are harmless, but others can lead people to miss their window of effectiveness entirely.
Myth: Plan B causes an abortion. This is false. Plan B prevents ovulation. It does not terminate a pregnancy. If you are already pregnant, Plan B will not work and will not harm the pregnancy.
Myth: Plan B works up to 5 days, so waiting is fine. While some data suggests partial effectiveness on days 4 and 5, this is not the approved use. The pill is designed for 72 hours, and effectiveness drops significantly after the first day. Treat 72 hours as the hard limit, not day 5.
Myth: Taking Plan B multiple times in one cycle is dangerous. It is not dangerous, but it is not a good birth control strategy. Frequent use causes irregular bleeding and can disrupt your cycle. Plan B is meant for emergencies, not as a regular contraceptive method.
Myth: Plan B works immediately. It does not. The pill needs time to affect your hormone levels. If you have sex again after taking Plan B but before your next period, you can still become pregnant from that later encounter.
What to Expect After Taking Plan B
Most people experience some side effects within a day or two of taking Plan B. Nausea is the most common, and some people vomit. If you vomit within 2 hours of taking the pill, contact a pharmacist — you may need a second dose.
Other common side effects include headache, breast tenderness, dizziness, and fatigue. These usually resolve within 24 to 48 hours. Your next period may come earlier or later than expected, and your flow may be lighter or heavier than usual.
If your period is more than 7 days late, take a pregnancy test. A negative test at that point is highly reassuring. If the test is positive, seek medical care to confirm the pregnancy and discuss your options.
When to See a Doctor
You should see a doctor if you are past the 72-hour window and need emergency contraception. The copper IUD is still an option up to 5 days after sex, and it is more effective than any pill.
You should also see a doctor if you have taken Plan B and experience severe abdominal pain, especially on one side. This can be a sign of an ectopic pregnancy, which is a medical emergency. While rare, ectopic pregnancy is a real risk with any contraceptive failure.
If you find yourself needing emergency contraception more than once in a single cycle or multiple times in a year, it is worth discussing long-term birth control options with a clinician. Emergency contraception is not designed for repeated use, and regular methods are far more effective at preventing pregnancy.
Frequently Asked Questions
Can I take Plan B after 72 hours?
It is not recommended, but some evidence suggests it may still have partial effect up to 120 hours. A copper IUD or ella may be better options at that point.
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 timing before ovulation is critical.
How soon after sex should I take Plan B?
Take it within 24 hours for the best chance of effectiveness. It remains approved for use up to 72 hours, but every hour matters.
Will Plan B work if I weigh over 200 pounds?
Evidence suggests it may be less effective at that weight. The copper IUD is the most reliable option regardless of body weight.
