Plan B works by delaying ovulation, and if ovulation has already happened, the pill has no way to prevent pregnancy. That single biological fact is the most common reason it fails. But timing isn’t the only factor. Body weight, vomiting, and certain medications can also reduce or eliminate its effectiveness.
How Plan B Actually Prevents Pregnancy
Plan B contains a synthetic hormone called levonorgestrel that delays or blocks the release of an egg from the ovary. It does this by suppressing the hormonal surge that triggers ovulation. According to FDA review data, the strongest clinical evidence supports this as the primary mechanism, with weak to speculative evidence for any effect after ovulation has already occurred.
This means Plan B has a narrow window of usefulness. It works when taken during the first half of your cycle, before your body has released an egg. Once ovulation happens, the expected pregnancy rate is the same whether you take Plan B or not. The pill also cannot end a pregnancy that has already started. Once a fertilized egg has implanted in the uterus, Plan B has no effect.
You May Have Already Ovulated
This is the number one reason Plan B fails, and it’s the hardest one to control. Most people don’t know exactly when they ovulate. If unprotected sex happened on the day of ovulation or shortly after, Plan B simply arrives too late to do its job. The egg is already out, and the drug has nothing left to delay.
Ovulation typically happens around the midpoint of a menstrual cycle, but it varies widely from person to person and even cycle to cycle. Stress, illness, irregular periods, and other factors can shift ovulation earlier or later than expected. If you happened to ovulate earlier than usual, Plan B taken even within the recommended window may not help because the biological opportunity it depends on has already passed.
Every Hour Matters
Plan B is most effective when taken as soon as possible after unprotected sex. Studies show it’s roughly 94% effective within the first 24 hours, but that number drops to about 58% by the 72-hour mark. After 72 hours, it’s generally not recommended at all.
That steep decline isn’t just about the drug wearing off. The longer you wait, the more likely your body has already begun or completed the ovulation process. A delay of even one day can mean the difference between suppressing the hormonal surge and missing it entirely.
Body Weight Reduces Effectiveness
Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason is straightforward: levonorgestrel blood levels were about 50% lower in people with a BMI of 30 after taking a standard dose, meaning the drug never reached the concentration needed to block ovulation.
Doubling the dose doesn’t solve the problem either. The same research team tested a double dose in a clinical trial of 70 participants with BMIs above 30 and weights of at least 176 pounds, and found it was not effective at preventing pregnancy in this group. For people with a higher BMI, a different emergency contraceptive (sold under the brand name ella) may be a better option. It also appears to be affected by weight, but not to the same degree.
A copper IUD, which can be placed by a healthcare provider within five days of unprotected sex, is the most effective emergency contraception regardless of body weight.
Vomiting Can Prevent Absorption
If you vomit within three hours of taking Plan B, your body may not have absorbed enough of the drug for it to work. CDC guidelines recommend taking another dose as soon as possible if this happens. Taking an anti-nausea medication beforehand can help if you’re prone to stomach upset, though this isn’t always practical in an emergency situation.
Certain Medications Interfere
Some prescription drugs and supplements speed up the liver enzymes that break down levonorgestrel, lowering its concentration in your blood before it can do its job. The FDA lists several that can reduce effectiveness:
- Seizure medications such as carbamazepine, phenytoin, oxcarbazepine, topiramate, and felbamate
- Certain antibiotics and antifungals, including rifampin and griseofulvin
- St. John’s wort, an herbal supplement commonly used for mood support
- Barbiturates and bosentan (used for pulmonary hypertension)
If you take any of these regularly, Plan B may not reach effective levels in your system. A copper IUD or ella (which uses a different active ingredient) may be more reliable alternatives, though ella can also be affected by some of the same drug interactions.
How to Know If Plan B Didn’t Work
Plan B commonly shifts your next period by a few days in either direction, so a slightly late period alone isn’t a reliable sign of failure. However, if your period hasn’t arrived within three weeks of taking the pill, take a pregnancy test. Home tests are accurate by that point because the hormone they detect has had enough time to build up to measurable levels.
Other early signs to watch for include unusual breast tenderness, nausea, or fatigue that persists beyond a few days after taking the pill. Plan B itself can cause short-term side effects like nausea, headache, and spotting, so it’s not always easy to tell the difference. The three-week pregnancy test is the most definitive next step.