The morning after pill can fail in several specific situations, and the most common one is timing: you’ve already ovulated. Because these pills work by delaying or preventing ovulation, they can’t do much once your body has already released an egg. Beyond that, your weight, certain medications, and even vomiting after taking the pill can all reduce its effectiveness.
After Ovulation, It Can’t Help
This is the single biggest reason the morning after pill doesn’t work. Both major types of emergency contraceptive pills, levonorgestrel (Plan B and generics) and ulipristal acetate (ella), prevent pregnancy by delaying or blocking ovulation. They do not prevent a fertilized egg from implanting, and they cannot interrupt a pregnancy that has already begun. So if you’ve already ovulated and the egg has been released, the pill has no mechanism left to stop conception.
The tricky part is that most people don’t know exactly when they ovulate. Ovulation typically happens around the middle of your cycle, but it can shift by several days depending on stress, sleep, illness, or cycle irregularity. If you happened to ovulate shortly before unprotected sex, the pill may have arrived too late to make a difference. This is also why the pill is less effective the longer you wait to take it: with each passing hour, you’re closer to ovulation or past it entirely.
Every Hour of Delay Matters
The morning after pill is most effective the sooner you take it. Within the first 72 hours, levonorgestrel-based pills reduce the risk of pregnancy by 87% to 90%, with a pregnancy rate of about 0.8%. Between 72 and 120 hours (three to five days), effectiveness drops to 72% to 87%, and the pregnancy rate rises to 1.8%. That’s still a meaningful reduction compared to no contraception at all, but significantly less reliable than taking it within the first day.
Ulipristal acetate maintains its effectiveness somewhat better over the five-day window, which is one reason it requires a prescription in many countries. Levonorgestrel-based options, available over the counter, lose potency more noticeably with each passing day. Either way, taking the pill within 24 hours gives you the best odds.
Body Weight Can Reduce Effectiveness
Your weight plays a real role in how well emergency contraception works. The American College of Obstetricians and Gynecologists notes that levonorgestrel-based pills may be less effective for people who are overweight (BMI of 25 to 29.9) or obese (BMI of 30 or higher). Ulipristal acetate also loses some effectiveness at higher weights, but it holds up better than levonorgestrel, which appears to lose efficacy at a lower weight threshold.
This doesn’t mean the pill is useless at a higher body weight. It still reduces pregnancy risk compared to taking nothing. But if you’re in this weight range and want the most reliable emergency option, a copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception regardless of body size, with a failure rate below 1%.
Vomiting Too Soon After Taking It
If you vomit within two hours of swallowing the pill, your body may not have absorbed enough of the medication for it to work. In that case, you need to take another dose. This is a straightforward fix if you act quickly, but it’s easy to overlook, especially since nausea is one of the pill’s own side effects. If you’re prone to nausea, eating a small amount of food before taking the pill or using an anti-nausea medication can help keep it down.
Certain Medications Interfere
Several drugs speed up how your liver breaks down hormones, which can reduce the amount of active medication in your system. These include rifampin (used for tuberculosis), griseofulvin (an antifungal), certain seizure medications, some antiretroviral drugs, and the herbal supplement St. John’s wort. All of these can lower the effectiveness of both levonorgestrel and ulipristal acetate.
If you’re taking any of these, a copper IUD is the recommended first-line emergency contraception because its effectiveness isn’t affected by other drugs. Some guidelines suggest doubling the levonorgestrel dose to 3 mg if that’s the only option available. Ulipristal acetate is specifically not recommended for people on enzyme-inducing medications.
Repeated Use in the Same Cycle
Taking the morning after pill more than once in a single menstrual cycle is not inherently dangerous, and the available evidence doesn’t show a clear increase in adverse outcomes from higher cumulative doses. However, repeated use within one cycle is a signal that the method isn’t serving you well as a primary strategy, and the overall evidence on safety is limited.
One notable finding: among people who became pregnant despite taking levonorgestrel emergency contraception, those who had used it multiple times in the same cycle had roughly 2.5 times the odds of an ectopic pregnancy (where the embryo implants outside the uterus) compared to those who used it once. The data behind this is from a single study and considered low certainty, but ectopic pregnancies are a serious medical situation, so it’s worth being aware of.
How to Know If It Worked
There’s no immediate way to tell. Your next period may come on time, a few days early, or a few days late. Some spotting or changes in flow are normal after taking emergency contraception. The reliable answer comes from a pregnancy test taken three weeks after the day you took the pill. Testing earlier than that can produce a false negative because pregnancy hormone levels may not yet be detectable.
If your period is more than a week late, that’s a reasonable prompt to test even before the three-week mark. And if you experience severe lower abdominal pain after a positive test, particularly if you used emergency contraception multiple times that cycle, seek medical attention promptly, as this can be a sign of ectopic pregnancy.