Several things can reduce or completely cancel out Plan B’s ability to prevent pregnancy: timing in your menstrual cycle, body weight, certain medications, herbal supplements, vomiting after taking the pill, and simply waiting too long. Plan B works by delaying ovulation, so anything that interferes with that single mechanism can undermine it.
How Plan B Works (and Why It Can Fail)
Plan B contains a synthetic hormone called levonorgestrel. Its primary job is to stop or delay your ovary from releasing an egg. If there’s no egg available, sperm can’t fertilize anything. That’s the entire mechanism. Plan B does not end a pregnancy that has already started, and it doesn’t create a barrier or kill sperm. This means if ovulation has already happened before you take it, Plan B has essentially nothing left to do.
This is the single biggest reason Plan B fails. If you’ve already ovulated, or if you’re so close to ovulating that the pill can’t delay it in time, the egg is already on its way. Sperm can survive inside the body for up to five days, so even if you had sex before ovulation, the egg and sperm can still meet. Unfortunately, there’s no easy way to know exactly where you are in your cycle unless you’ve been tracking it closely.
Waiting Too Long to Take It
Plan B is 81 to 90 percent effective at preventing pregnancy, but that number drops the longer you wait. It works best within the first 24 hours after unprotected sex. Effectiveness continues to decline through 48 and 72 hours. You can still take it between 73 and 120 hours (three to five days), but at that point it’s significantly less reliable.
The reason is straightforward: every hour you wait is an hour closer to possible ovulation. The pill needs time to deliver enough hormone to block your ovary’s signal to release an egg. The sooner you take it, the better the odds that it intervenes before that window closes.
Body Weight
Your weight can meaningfully reduce how well Plan B works. The American College of Obstetricians and Gynecologists notes that levonorgestrel may be less effective for women who are overweight (BMI of 25 to 29.9) or obese (BMI of 30 or higher). The hormone gets distributed across more body tissue, which lowers its concentration in your blood and weakens its ability to block ovulation.
This doesn’t mean Plan B is worthless at higher body weights. It still offers some protection, and medical guidelines say it shouldn’t be withheld based on weight alone. But if your BMI is 25 or above, you may want to ask a pharmacist or provider about a copper IUD as emergency contraception, which is the most effective option regardless of weight. Another emergency contraceptive pill, sold under the brand name ella, retains its effectiveness at a higher weight threshold than Plan B does.
Medications That Speed Up Metabolism
Certain prescription drugs break down levonorgestrel in your liver faster than normal, reducing the amount circulating in your blood. These are called enzyme inducers because they ramp up a specific liver enzyme (CYP3A4) that metabolizes the hormone. The UK’s medicines regulatory agency warns that this interaction can meaningfully reduce emergency contraceptive effectiveness.
The main drug categories involved:
- Epilepsy medications: barbiturates, primidone, phenytoin, carbamazepine
- Tuberculosis medications: rifampin, rifabutin
- HIV medications: ritonavir, efavirenz (efavirenz alone reduces levonorgestrel blood levels by about 50%)
- Antifungal medications: griseofulvin
One critical detail: these enzyme effects don’t stop the day you quit the medication. Elevated enzyme levels can persist for up to four weeks after you stop taking the drug. So even if you recently discontinued one of these medications, Plan B may still be compromised. If you take any of these, a copper IUD is a more reliable emergency contraception option.
St. John’s Wort
This herbal supplement, commonly taken for mood support, induces the same liver enzymes as the prescription drugs listed above. It speeds up how quickly your body clears levonorgestrel, lowering the effective dose in your system. Some clinical guidance suggests that women taking St. John’s Wort may need a higher dose of levonorgestrel (2.25 mg instead of the standard 1.5 mg) to compensate, though this isn’t something to adjust on your own without guidance from a pharmacist.
Because St. John’s Wort is sold over the counter and often not thought of as a “real” medication, many people don’t realize it can interfere with hormonal contraception. If you take it regularly, mention it any time you’re discussing emergency contraception options.
Vomiting After Taking the Pill
If you throw up within two hours of swallowing Plan B, your body may not have absorbed enough of the hormone for it to work. In that case, you need to take another dose. This applies to any cause of vomiting, whether it’s a stomach bug, motion sickness, or nausea from the pill itself (which is a known side effect).
If nausea is a concern, taking Plan B with a small amount of food can help. If you’ve vomited and can’t keep a second dose down, contact a pharmacist about alternative emergency contraception methods.
Things That Don’t Cancel Out Plan B
Some common worries aren’t actually backed by evidence. Alcohol doesn’t interfere with levonorgestrel’s effectiveness. Neither does ibuprofen, acetaminophen, or most common antibiotics (the exception being rifampin, listed above, which is used for tuberculosis, not routine infections). Food doesn’t block absorption.
Starting or resuming regular hormonal birth control pills after taking Plan B also doesn’t cancel it out. CDC guidelines confirm that progestin-only birth control pills can be started immediately after taking levonorgestrel-based emergency contraception. You don’t need to wait. The one exception is if you took ella (ulipristal acetate) instead of Plan B. In that case, you should wait five days before starting hormonal birth control, because the two can interfere with each other.
Having sex again after taking Plan B is another concern worth addressing. The pill only covers the sex you already had. It does not provide ongoing protection. If you have unprotected sex again after taking it, even later the same day, you’re at risk of pregnancy from that new exposure.