Plan B works by delaying ovulation, and if your body has already released an egg, the pill has little to no effect. That single biological fact accounts for most failures, but timing isn’t the only reason. Your weight, other medications you take, and even vomiting shortly after swallowing the pill can all reduce or eliminate its effectiveness.
Ovulation Timing Is the Biggest Factor
Plan B contains a synthetic hormone called levonorgestrel that works by suppressing the hormonal signal your brain sends to trigger ovulation. If that signal hasn’t fired yet, Plan B can delay or block egg release entirely, preventing sperm from ever meeting an egg. But once your body has already begun the ovulation process, the natural hormone surge is too strong for Plan B to override.
This isn’t a small reduction in effectiveness. Studies comparing Plan B to placebo in women who had already ovulated found virtually identical pregnancy rates: about 33% with Plan B versus 37.5% without it. In other words, taking Plan B after ovulation provided no meaningful protection at all. The pill does not prevent fertilization, does not block implantation, and does not affect an established pregnancy. Its only job is stopping the egg from being released in the first place.
The tricky part is that most people don’t know exactly when they ovulate. Ovulation typically happens around day 14 of a 28-day cycle, but it can shift by several days depending on stress, sleep, illness, or naturally irregular cycles. If you had unprotected sex close to ovulation, Plan B may simply arrive too late to do its job.
Your Weight Can Reduce Effectiveness
Plan B becomes less effective at higher body weights. Planned Parenthood notes that if you weigh more than 165 pounds, levonorgestrel-based emergency contraception won’t work as well. The reason is pharmacological: levonorgestrel is distributed through your body based on your size, so a fixed 1.5 mg dose produces lower blood concentrations in someone who weighs more. Lower blood levels mean a weaker suppression of the ovulation signal.
If your weight is above that threshold, an alternative called ulipristal acetate (sold as Ella) is a better option. In a large clinical trial of over 2,200 women, ulipristal acetate outperformed levonorgestrel overall, and the gap widened the longer women waited to take it. Among women who took emergency contraception between 72 and 120 hours after sex, all three pregnancies in the study occurred in the levonorgestrel group. None occurred in the ulipristal acetate group. Ella also maintains more of its effectiveness at higher body weights, though it does require a prescription.
A copper IUD, inserted within five days of unprotected sex, remains the most effective emergency contraceptive regardless of weight.
Certain Medications Break It Down Faster
Your liver processes levonorgestrel using a specific enzyme pathway. Medications that speed up that pathway cause your body to clear Plan B from your bloodstream faster, sometimes dramatically so. One HIV medication, efavirenz, cuts levonorgestrel blood levels by roughly 50%.
Drugs known to reduce Plan B’s effectiveness include:
- Epilepsy medications such as phenytoin, carbamazepine, and barbiturates
- Tuberculosis drugs such as rifampicin and rifabutin
- HIV medications such as ritonavir and efavirenz
- Antifungal medications such as griseofulvin
- St. John’s Wort, an herbal supplement commonly used for mood support
This interaction doesn’t disappear the day you stop taking these medications. Elevated enzyme levels can persist for up to four weeks after stopping the drug. The UK’s medicines regulator advises that women in this situation either use a copper IUD for emergency contraception or take a double dose of levonorgestrel (two packs, totaling 3 mg) to compensate for the faster breakdown.
Vomiting Can Prevent Absorption
Plan B needs to be absorbed through your digestive tract to reach your bloodstream. If you vomit within two hours of taking the pill, you may not have absorbed enough of the drug for it to work. The FDA’s prescribing information recommends contacting a healthcare provider about taking a second dose if this happens. Nausea is one of Plan B’s more common side effects, which makes this a real concern for some people.
Waiting Too Long Reduces the Odds
Plan B is marketed as effective up to 72 hours after unprotected sex, but its effectiveness drops with every passing hour. This isn’t just because the drug wears off. The longer you wait, the more likely your body has already begun the ovulation process that Plan B can no longer stop. Taking it within the first 24 hours gives the best results. By the 72-hour mark, effectiveness has already declined noticeably, and beyond 72 hours, levonorgestrel performs poorly. In clinical trials, all pregnancies among women who took emergency contraception between 72 and 120 hours occurred in the levonorgestrel group, not the ulipristal acetate group.
It Won’t End an Existing Pregnancy
If a fertilized egg has already implanted in your uterus, Plan B will not end the pregnancy. The FDA states clearly that Plan B has no direct effect on fertilization, implantation, or maintenance of an existing pregnancy. This means if you were already pregnant before the unprotected sex that prompted you to take Plan B, or if fertilization and implantation happened before the drug took effect, the pill will not change that outcome. A positive pregnancy test two to three weeks after taking Plan B means the pill did not work, not that it caused harm to the pregnancy.