The single most reliable sign that Plan B didn’t work is a missed period. If your period hasn’t arrived within three weeks of taking the pill, you should take a pregnancy test. There is no way to feel whether Plan B succeeded in the hours or days after swallowing it, and many of its normal side effects overlap with early pregnancy symptoms, which makes the waiting period genuinely stressful. Understanding why Plan B sometimes fails and what to watch for can help you figure out your next steps with less guesswork.
How Plan B Works, and Why That Matters for Failure
Plan B’s active ingredient, levonorgestrel, works almost entirely by delaying or blocking ovulation. If you take it before your body has released an egg, the drug postpones that release long enough for sperm to die off. A review of levonorgestrel emergency contraception confirmed that the drug acts by postponing or inhibiting ovulation and has little to no effect after ovulation has already occurred. If it doesn’t succeed in blocking ovulation, the resulting chance of conception is about the same as if you’d had unprotected sex during your most fertile window without taking anything at all.1Contraception. Effect of levonorgestrel emergency contraception on implantation and fertility: A review
This is the critical point most people don’t realize: Plan B is not an all-purpose safety net that works regardless of timing. It is a narrow-window ovulation blocker. Research found that levonorgestrel disrupts the ovulatory process in about 93% of cycles when the dominant follicle is still in an early growth phase, roughly 12 to 17 millimeters in diameter. The researchers concluded that this mechanism fully accounts for both the drug’s effectiveness and its failure rate.2PubMed. Pituitary-ovarian function following the standard levonorgestrel emergency contraceptive dose or a single 0.75-mg dose given on the days preceding ovulation Once the hormonal surge that triggers ovulation is already underway, levonorgestrel loses its ability to stop the egg from being released. So whether Plan B works for you depends heavily on where you are in your cycle at the moment you take it.
The Signs to Watch For
Because Plan B doesn’t produce a clear “it worked” signal, you’re essentially watching for the absence of pregnancy rather than for proof of success. Here’s what to pay attention to in the days and weeks after taking it.
Your period is the main indicator. Plan B frequently shifts your cycle by a few days in either direction. Your period might come a little early, a little late, or roughly on time. A shift of up to a week isn’t unusual and doesn’t mean anything went wrong. But if your period is more than a week late, or if three weeks have passed since you took Plan B and you still haven’t had any bleeding, that’s the point where you need a pregnancy test.3PubMed Central. Emergency contraception
Bleeding that does show up can look different from your usual period. Some people get lighter bleeding, heavier bleeding, or spotting between periods after taking Plan B. Irregular bleeding on its own isn’t a sign of failure. It’s a common hormonal side effect of the high dose of levonorgestrel. The bleed you’re looking for is one that resembles a real period in timing and flow, confirming that pregnancy didn’t occur.
Telling Apart Plan B Side Effects From Early Pregnancy
This is where things get genuinely confusing. Plan B delivers a large dose of synthetic progesterone, and early pregnancy also causes a surge in progesterone-related hormones. The overlap in symptoms is almost comically unhelpful for someone trying to figure out whether the pill worked.
Common Plan B side effects include nausea, breast tenderness, fatigue, headaches, and mild cramping. Early pregnancy symptoms include nausea, breast tenderness, fatigue, headaches, and mild cramping. You can see the problem. In the first week or two after taking Plan B, these symptoms tell you essentially nothing about whether you’re pregnant.
There are a few rough timing cues that can help. Plan B side effects tend to peak within the first day or two and then fade. If nausea hits you a few hours after taking the pill and is gone by the next morning, that’s the drug. If nausea appears for the first time two or three weeks later and lingers, that’s more concerning. Similarly, breast tenderness from Plan B usually resolves within a few days, while pregnancy-related breast tenderness tends to start later and persist or intensify.
But these timing differences are rough guidelines, not diagnostic tools. The only way to know for sure is a pregnancy test. No combination of symptoms can substitute for one.
When and How to Take a Pregnancy Test
Home pregnancy tests detect a hormone called hCG, which the body starts producing after a fertilized egg implants in the uterine lining. Implantation typically happens six to twelve days after fertilization, and it takes a couple more days for hCG levels to rise enough for a test to detect.
The practical upshot: testing too early will give you a false negative. If you take a pregnancy test three days after unprotected sex, it will almost certainly read negative even if you are going to become pregnant, because hCG simply isn’t in your system yet. The earliest a test can reliably detect pregnancy is around ten to fourteen days after the sex you’re worried about. For the most accurate result, wait until your period is actually late. If you haven’t had a period within 21 days of taking emergency contraception, that’s the recommended point for evaluation.3PubMed Central. Emergency contraception
If you get a negative result but your period still hasn’t come after another week, test again. Occasionally hCG rises slowly or implantation happens on the later end of the window. A second test a week after the first catches most of these cases.
Why Plan B Fails
The most common reason Plan B doesn’t work is timing relative to ovulation. If you’ve already ovulated, or if the hormonal cascade that triggers ovulation is already in progress, levonorgestrel can’t stop the egg from being released. The drug is most effective when taken well before ovulation, during the early-to-mid follicular phase. Once the hormonal surge that precedes ovulation has begun, levonorgestrel loses efficacy.4PubMed Central. Ulipristal Acetate (ella): A Selective Progesterone Receptor Modulator For Emergency Contraception
The trouble is that most people don’t know exactly where they are in their cycle at any given moment. Ovulation doesn’t happen on a perfectly predictable schedule, especially for people with irregular periods. You might think you’re in a “safe” part of your cycle and actually be closer to ovulation than you realize. This biological uncertainty is baked into the failure rate.
Speed also matters. Plan B is more effective the sooner you take it after unprotected sex, and the general advice is to take it as quickly as possible.5PubMed. Effectiveness of emergency contraceptive pills between 72 and 120 hours after unprotected sexual intercourse The conventional window is 72 hours, though some effectiveness persists up to 120 hours. But effectiveness drops with each passing day, and much of the decline probably reflects the fact that the longer you wait, the closer you are to ovulation happening on its own.
Body Weight
There has been significant debate about whether body weight affects Plan B’s effectiveness. The concern is biologically plausible: levonorgestrel is a fixed dose, and a higher body mass means lower drug concentration in the blood. Some regulatory agencies have discussed the question of whether morning-after pills should carry weight-related warnings.6PubMed Central. Rethink weight limits on morning-after pill The European Medicines Agency at one point added a note about reduced efficacy in people weighing over 75 kilograms (about 165 pounds), though the FDA did not follow suit, concluding the evidence was insufficient to add a label warning.
The practical takeaway is that if you weigh more, Plan B might be less effective for you, but the evidence isn’t strong enough for regulators to agree on exactly where the cutoff falls or how steep the decline in effectiveness is. If this is a concern, other emergency contraception options with better evidence across weight ranges are worth knowing about.
Warning Signs That Need Immediate Medical Attention
Plan B does not cause ectopic pregnancy, but it also doesn’t prevent one. An ectopic pregnancy occurs when a fertilized egg implants outside the uterus, most commonly in a fallopian tube. This is a medical emergency that can become life-threatening if the tube ruptures. A documented case report described a patient who presented with abdominal pain and vaginal spotting after using Plan B and was found to have an ectopic pregnancy in her right fallopian tube. The case underscored the importance of educating patients about the possible outcomes after emergency contraception use, including both failure and the potential for ectopic pregnancy.7PubMed. Ectopic Pregnancy After Plan B Emergency Contraceptive Use
Symptoms to watch for include sharp or stabbing pain on one side of your lower abdomen, pain in your shoulder tip (which sounds strange but results from internal bleeding irritating the diaphragm), dizziness or fainting, and vaginal bleeding that’s different from a normal period, often darker or more watery. Any of these symptoms in the weeks after taking Plan B warrant an immediate trip to the emergency room, not a wait-and-see approach. Ectopic pregnancies are rare but are always treated as urgent.
Alternatives With Better Efficacy
If you’re reading this article because Plan B may have failed, or because you want a backup plan that’s more reliable next time, two alternatives are worth knowing about. Both are available with a prescription or through a healthcare provider.
Ulipristal Acetate
Sold under the brand name ella in the United States, ulipristal acetate is a different type of emergency contraceptive pill. It works through a different mechanism than levonorgestrel and has one major advantage: it remains effective closer to ovulation. Research found that ulipristal acetate administered just before ovulation significantly delayed the release of the egg by an average of four and a half days, whereas levonorgestrel given at the same point did not significantly delay it. Ulipristal acetate stayed effective even on the day of the hormonal surge that triggers ovulation, while levonorgestrel lost efficacy once that surge had begun.4PubMed Central. Ulipristal Acetate (ella): A Selective Progesterone Receptor Modulator For Emergency Contraception
Ella requires a prescription, which adds a logistical step that Plan B doesn’t have. But for someone who suspects they may be close to ovulation, or who is taking emergency contraception more than 24 hours after unprotected sex, ella offers a meaningful edge. It also maintains its effectiveness over a full five-day window more consistently than levonorgestrel does.
The Copper IUD
The most effective form of emergency contraception isn’t a pill at all. A copper IUD inserted within five days of unprotected sex has a failure rate of roughly 0.09%, based on a combined analysis of over 8,500 insertions that produced only eight pregnancies.8PubMed Central. The efficacy of intrauterine devices for emergency contraception and beyond: a systematic review update That failure rate is dramatically lower than any emergency contraceptive pill. The copper IUD works by creating an environment in the uterus that is hostile to both sperm and implantation, so it remains effective even after ovulation has occurred.
The tradeoff is access. Getting a copper IUD requires a healthcare visit and an insertion procedure, which isn’t always possible within the five-day window, especially on weekends or in areas with limited provider availability. But if you can get one placed in time, it doubles as highly effective ongoing contraception for up to ten or twelve years. For someone who has already experienced a Plan B failure and wants to avoid repeating the experience, the copper IUD addresses both the immediate concern and the long-term one.
What Happens If You Confirm You’re Pregnant
If a pregnancy test comes back positive after taking Plan B, the drug has not harmed the pregnancy. Levonorgestrel is not an abortion pill and does not disrupt an existing pregnancy. Research on outcomes after emergency contraception failure has not found an increased risk of birth defects or complications from the drug itself. A positive test simply means that ovulation wasn’t blocked in time and fertilization and implantation proceeded normally.
At that point, your options depend on your circumstances, your location, and your preferences. Scheduling an appointment with a healthcare provider is the logical first step regardless of what you decide to do. A provider can confirm the pregnancy with a blood test or ultrasound, estimate how far along you are, and discuss your options. If you’re experiencing any of the warning symptoms described earlier, particularly one-sided abdominal pain or unusual bleeding, mention those immediately so ectopic pregnancy can be ruled out.
Vomiting After Taking Plan B
One scenario that people don’t always think about: if you vomit within two hours of taking Plan B, the pill may not have been fully absorbed. Nausea is one of the most common side effects of levonorgestrel, so this isn’t a rare situation. If you throw up shortly after taking it, the standard recommendation is to take another dose. Some providers suggest taking an anti-nausea medication before the second dose to prevent the same thing from happening again. If vomiting happens more than two hours after taking the pill, enough of the drug has likely been absorbed for it to have its intended effect.
This is a failure mode that’s entirely preventable but easy to overlook in the stress of the moment. If you know from experience that you’re prone to nausea from medications, eating a small amount of food before taking Plan B and having an anti-nausea option on hand can help keep the pill down long enough to work.