The only definitive way to know Plan B worked is getting your period. If your next period arrives, even if it’s a little early, late, or different from usual, that’s the clearest sign you’re not pregnant. If your period doesn’t come within three weeks of taking the pill, take a pregnancy test.
That three-week window can feel long, and the side effects of Plan B overlap with early pregnancy symptoms in frustrating ways. Here’s how to make sense of what your body is doing in the days and weeks after taking it.
Your Period Is the Most Reliable Signal
Plan B works by delaying or preventing ovulation, so your cycle will often shift after taking it. Your next period may come earlier than expected, later than expected, or right on time. It may be heavier, lighter, or spottier than normal. All of these variations are common and don’t indicate a problem.
A period that shows up in any form is a strong sign the pill did its job. The concern starts when your period is significantly late. Planned Parenthood recommends taking a pregnancy test if you haven’t gotten your period within three weeks of taking Plan B. At that point, a home pregnancy test is reliable enough to give you a clear answer.
Why You Can’t Trust Symptoms Alone
Plan B delivers a large dose of a synthetic hormone, and that hormone surge causes side effects that look a lot like early pregnancy: nausea, fatigue, breast tenderness, mood swings, spotting. Many people experience some combination of these in the first few days after taking the pill.
Early pregnancy causes nearly identical symptoms for the same reason: rising hormone levels. So feeling nauseous or exhausted a week after taking Plan B tells you almost nothing about whether it worked. The reverse is also true. Feeling completely fine doesn’t guarantee the pill succeeded. Some people who become pregnant have no early symptoms at all. The only way to confirm the outcome is a pregnancy test, ideally taken at least two weeks after the encounter to avoid a false negative.
Timing Matters More Than You Think
Plan B is 81 to 90 percent effective overall, but that number depends heavily on when you take it. The sooner you take it after unprotected sex, the better it works. It’s most effective within the first 24 hours and continues to offer protection up to 72 hours. You can still take it between 73 and 120 hours (three to five days), but effectiveness drops noticeably in that window.
Timing within your menstrual cycle also plays a major role. Plan B works by pulling the emergency brake on ovulation. If your body has already released an egg, the pill can’t undo that. This means Plan B is least effective when taken during or just after ovulation, which typically happens around the middle of your cycle. If you track your cycle and know you were near ovulation, that context is worth keeping in mind as you wait for your period.
Body Weight Affects How Well It Works
Research shows that Plan B becomes less effective at higher body weights. The decline starts at around 70 kg (about 155 pounds), and studies suggest the standard dose has essentially no efficacy for people weighing 80 kg (about 176 pounds) or more. The reason is straightforward: higher body weight means the hormone reaches lower peak concentrations in the blood. People with a BMI of 30 or above had roughly four times the risk of pregnancy compared to those with a BMI under 25 when using the standard dose.
If this applies to you, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception regardless of weight. Another option, a prescription pill called ella, maintains better efficacy at higher weights than Plan B does. UK clinical guidelines also suggest doubling the levonorgestrel dose for people at higher weights, though this isn’t yet standard practice in the United States.
When to Take a Pregnancy Test
A pregnancy test detects a hormone your body only produces after a fertilized egg implants, which takes at minimum about ten days after conception. Testing before that point will almost certainly give you a negative result whether you’re pregnant or not. For the most reliable reading, wait at least two weeks after the encounter before testing. Three weeks is even better and lines up with the timeline for a missed period.
If you get a negative result but your period still hasn’t arrived after another week, test again. Occasionally, implantation happens on the later end of the window, and an early test can miss it.
Warning Signs That Need Immediate Attention
In rare cases, a pregnancy can implant outside the uterus, most commonly in a fallopian tube. This is called an ectopic pregnancy, and it’s a medical emergency. Plan B does not cause ectopic pregnancies, but if the pill fails and pregnancy occurs, ectopic implantation is one possible outcome.
The early warning signs are light vaginal bleeding combined with pelvic pain, sometimes with shoulder pain or a strong urge to have a bowel movement. If the tube ruptures, symptoms escalate to severe abdominal pain, heavy internal bleeding, extreme lightheadedness, or fainting. These symptoms require emergency care immediately, regardless of whether you’ve taken emergency contraception.
A Realistic Timeline for Peace of Mind
In the first one to three days after taking Plan B, expect possible nausea, headache, fatigue, or spotting. These are side effects of the pill itself and don’t indicate success or failure. Over the next one to three weeks, you’re essentially waiting for your period. It may arrive on schedule, a few days early, or a few days late. Any of these outcomes is normal.
If three weeks pass with no period, take a home pregnancy test. A negative result at that point is highly reliable. A positive result means the pill didn’t prevent pregnancy, and your next step is scheduling an appointment with a healthcare provider to confirm the result and discuss your options.