Key Takeaways Up Front
Plan B (levonorgestrel) is designed to prevent pregnancy primarily by delaying or inhibiting ovulation. For most people, its effects resolve within 24 to 48 hours, and baseline fertility typically returns quickly after that. There is no universal clock for when Plan B "stops working," because effectiveness depends on where you are in your cycle, body weight, whether vomiting occurs, and whether another dose is taken too soon. Ongoing research suggests it remains an option for many people who are not pregnant, but timing and access can change. The following sections clarify mechanisms, timelines, and practical considerations using current evidence.
What Plan B Is and How It Works
Plan B is a progestin‑only emergency contraceptive containing levonorgestrel. It is not the same as the abortion pill (mifepristone and misoprostol) and does not terminate an established pregnancy. Instead, its primary mode is preventing or delaying ovulation so that sperm cannot meet an egg. It may also thicken cervical mucus and alter the endometrium, but preventing fertilization is the dominant pathway. Because it does not disrupt an implanted pregnancy, it is not effective once pregnancy is established.
Primary Mechanism: Ovulation Delay or Inhibition
By suppressing or shifting the luteinizing hormone (LH) surge, levonorgestrel can delay ovulation until several days after the pill is taken. If ovulation has already occurred before taking Plan B, the likelihood of pregnancy remains high because fertilization may still happen. This is why earlier use—closer to unprotected sex—leads to better effectiveness. The drug’s half‑life is roughly 29 hours, and most people clear it within 2–3 days, after which normal menstrual cycles typically resume.
How Long Plan B Typically Takes to Work
Plan B is designed to act quickly, with ovulation‑blocking effects often occurring within hours. Most people return to their usual fertility patterns within 24–48 hours after taking it. If vomiting occurs within two hours of taking the dose, the hormone may not absorb fully; in that case, another dose is advised. Effectiveness does not depend on a precise "expiration" point, but on whether ovulation was delayed before the sperm and egg could meet. Once a menstrual period starts, Plan B is no longer necessary for that episode, and a new episode is required for a separate act of unprotected sex.
When Effectiveness May Decline or Seem to Stop Working
No emergency contraceptive is 100% effective. Plan B’s success declines as ovulation approaches or occurs. If a person is already in the luteal phase of their cycle (after ovulation), Plan B is unlikely to prevent pregnancy. Body weight also matters: evidence suggests reduced effectiveness at higher body weights, though it still offers some benefit. Additional factors include drug interactions (such as certain anticonvulsants or St. John’s Wort) and gastrointestinal issues that impair absorption. These circumstances can make a given dose less effective, but they do not mean Plan B becomes active at a set "time limit."
Notable Details That Affect How Long It Works
| Attribute | Verified Detail | Source Type |
|---|---|---|
| Primary active ingredient | Levonorgestrel 1.5 mg | Label/Regulatory |
| Typical half‑life | ~29 hours | Pharmacology references |
| Recommended timeframe for best efficacy | Within 72 hours (sooner is better) | Guidelines |
| Reduced effectiveness at higher body weight | Evidence of lower efficacy above certain BMI/weight thresholds | Research studies |
| Not effective after implantation | Does not terminate established pregnancy | Clinical consensus |
Signs That Plan B Has "Worn Off" or Been Less Successful
There is no medical test that says Plan B has stopped working; instead, people infer this from their menstrual cycle. If a period arrives on time and is of usual flow, Plan B was likely effective for that episode. If a period is delayed by a week or more, a pregnancy test is recommended regardless of when Plan B was taken. Spotting or lighter bleeding can occur after use but does not confirm prevention. Ongoing, consistent protection requires a fresh plan for each act of unprotected sex.
Practical Considerations and Alternatives
Understanding when Plan B might not be enough helps people choose the next step. Copper IUDs are the most effective form of emergency contraception and can be used up to 120 hours after unprotected sex, while ulipristal acetate is another pill option available within 120 hours for those who meet eligibility criteria. If you vomit within two hours of taking Plan B, repeat the dose. Drug interactions and body weight can affect outcomes, so discussing options with a healthcare provider is valuable. Regular contraception remains the most reliable way to prevent unintended pregnancy day to day.
Quick Comparison of Emergency Contraception Options
- Plan B (levonorgestrel): effective up to 72 hours; sooner use increases success; reduced efficacy at higher body weight.
- Ulipristal acetate: effective up to 120 hours; requires prescription in many places; works later in the cycle than levonorgestrel.
- Copper IUD: most effective emergency option; can be inserted up to 120 hours; also provides ongoing contraception.
Common Questions and Clarity Points
Because Plan B is time‑sensitive in terms of when you take it relative to ovulation, rather than a fixed "expiration" after which it stops working, people often misunderstand how long it lasts. It does not remain in the body to block future acts of sex; each episode of unprotected sex may require its own evaluation. Effectiveness is not a simple on/off switch but a matter of probability shaped by many factors. Clear information about timing, body weight, cycle phase, and alternatives supports informed decisions.
Bottom Line
Plan B typically works quickly, with effects tapering within 24–48 hours after use. There is no fixed point at which Plan B suddenly "stops working," but its ability to prevent pregnancy decreases if ovulation has already occurred, body weight is higher, or vomiting impairs absorption. Being aware of these factors and knowing your options—including the copper IUD and ulipristal acetate—helps you make the best choice for your situation. For ongoing protection, consistent contraception and healthcare guidance are the most reliable paths.