Many people ask whether Plan B is 100 percent effective after unprotected sex or contraceptive failure. The short answer is no, Plan B does not guarantee complete prevention of pregnancy in every case.
Below you will find a detailed, scannable overview of how Plan B works in real-world conditions, how effectiveness varies by timing and body weight, and what steps to take next if you are concerned about an ongoing risk of pregnancy.
| Metric | Typical Use | Perfect Use | Key Condition |
|---|---|---|---|
| Effectiveness at preventing pregnancy | About 61% | Up to 84% | Used within 72 hours, no vomiting or meds interaction |
| Time window for use | Effective up to 72 hours, best within 24 | Effective up to 120 hours (5 days) | Efficacy decreases with each passing hour |
| Body weight impact | Lower effectiveness above 154–165 lbs | Considered less reliable above 165–180 lbs | Consider a copper IUD for higher BMI individuals |
| Vomiting after dose | Risk of reduced absorption | Not fully protective | Contact a clinician if vomiting occurs within 3 hours |
How Plan B Works in the Body
Plan B contains a high dose of levonorgestrel, a progestin that primarily delays or prevents ovulation.
If ovulation has already occurred, Plan B is much less likely to prevent pregnancy, which is why it is not 100 percent effective.
Because the drug relies on hormonal timing, effectiveness drops significantly the closer you are to the next scheduled ovulation.
Understanding this biological mechanism helps explain why two people can take the same dose but have different outcomes.
Timing and Real-World Effectiveness
Immediate use after contraceptive failure
Taking Plan B within a few hours of unprotected sex offers the best chance of preventing an unwanted pregnancy.
Delayed use up to 120 hours
While the medication can be used up to five days later, the probability of success declines substantially over time.
Interaction with medications
Drugs like certain anticonvulsants, St. John’s wort, and some HIV medications can reduce how well Plan B works.
Body Weight and Dosing Considerations
Clinical studies show that people with higher body weight may have lower circulating levels of levonorgestrel after taking Plan B.
For individuals above approximately 154–165 lbs, the standard 1.5 mg dose may be less reliable.
Some guidelines suggest that people over 176–180 lbs consider a copper IUD as a more dependable option.
What to Do After Taking Plan B
- Use a backup method, such as condoms, for the remainder of your current cycle.
- Take a pregnancy test if your period is more than one week late.
- Contact a healthcare provider to discuss ongoing contraception options.
- Seek emergency medical care if you experience severe abdominal pain or signs of an ectopic pregnancy.
Choosing Future Contraception
Relying on emergency contraception repeatedly is not ideal for ongoing sexual health or cycle control.
Long-acting methods like IUDs or implants provide much higher effectiveness and continuous protection.
Speaking with a clinician can help you find a solution tailored to your lifestyle, medical history, and family planning goals.
FAQ
Reader questions
Can Plan B fail even if taken within 24 hours? Yes, because it primarily delays ovulation and does not affect an already implanted embryo, it can still fail in certain situations. Does vomiting after taking Plan B reduce its effectiveness?
Yes, vomiting within three hours of ingestion may prevent full absorption, and you should contact a clinician for guidance.
Is Plan B less effective for people with higher body weight?
Yes, individuals above roughly 154–165 lbs may experience reduced protection and should discuss alternatives with a healthcare provider.
What should I do if my period does not arrive after using Plan B?
Take a pregnancy test about three weeks after taking the dose, and consult a medical professional if your period is more than one week late.