The most reliable sign that Plan B worked is getting your period. It may come on time, a few days early, or up to a week late. If your period arrives within that window, the pill almost certainly did its job. If your period is more than seven days late, that’s the clearest signal something may not have worked, and it’s time to take a pregnancy test.
There’s no immediate physical sensation that confirms Plan B was effective. The pill works silently, primarily by delaying or preventing ovulation, so your body won’t give you an obvious “it worked” signal right away. What you can do is watch for a few key markers over the next few weeks.
What to Watch for in the First Few Days
After taking Plan B, you may notice side effects like nausea, fatigue, headaches, breast tenderness, or cramping. Some people also experience light spotting. These are all normal reactions to the large dose of hormone in the pill and are not signs of pregnancy or failure. They’re also not confirmation the pill worked. They simply mean your body is processing the medication.
Spotting, in particular, can cause confusion because it looks similar to implantation bleeding (light bleeding that sometimes happens in very early pregnancy). The key difference is timing. Plan B spotting typically happens within the first few days of taking the pill. Implantation bleeding, if it occurs at all, usually shows up 10 to 14 days after conception. If you notice bleeding that’s heavier than spotting or that starts after you’ve already missed a period, contact a healthcare provider.
Your Period Is the Real Answer
Plan B delivers a concentrated dose of a synthetic hormone that delays ovulation. Because it disrupts your normal hormonal cycle, it commonly shifts the timing of your next period. Your period might show up a few days earlier than expected, or it could be delayed. Both are normal. The flow itself may also be lighter or heavier than usual.
The threshold to remember is seven days. If your period is more than a week later than you expected, take a home pregnancy test. At that point, enough time has passed for a test to give you a meaningful result.
When a Pregnancy Test Becomes Accurate
Home pregnancy tests detect a hormone your body only produces after a fertilized egg implants in the uterus. That process takes time, which is why testing too early gives unreliable results. For the most dependable reading, wait at least 14 days after the unprotected sex that prompted you to take Plan B. Testing at 21 days, or after a missed period, is even more accurate.
Early detection tests may pick up a pregnancy as soon as 12 days after unprotected sex, but a negative result at that point doesn’t guarantee you’re not pregnant. If you get a negative test but your period still hasn’t arrived, test again a few days later.
Factors That Affect Whether Plan B Works
Plan B is between 81% and 90% effective at preventing pregnancy, and timing is the biggest variable. It works best when taken as soon as possible after unprotected sex, ideally within 24 hours. You can take it up to 72 hours (three days) afterward, and it can still offer some protection in the 73 to 120 hour window, though effectiveness drops noticeably by then.
Body weight may also play a role, though the research is mixed. Some studies have found that Plan B is less effective in people who weigh over 165 pounds, with some data suggesting up to double the risk of pregnancy compared to people at lower body weights. Other studies haven’t confirmed that finding. If weight is a concern, a copper IUD inserted as emergency contraception is the most effective option at any body weight, and a provider can discuss alternatives like ella, a different emergency contraceptive pill.
One often-overlooked factor: if you vomited shortly after taking the pill, it may not have been absorbed. If vomiting happens within about 15 minutes of swallowing Plan B, or if you can see the intact pill in your vomit, you likely need to take another dose. After about an hour, the medication has generally moved past your stomach, so vomiting at that point shouldn’t affect its efficacy.
Signs the Pill May Not Have Worked
A missed period is the primary warning sign. Beyond that, early pregnancy symptoms can start showing up a few weeks after conception. These include persistent nausea (especially in the morning), breast tenderness that doesn’t go away, unusual fatigue, and frequent urination. The tricky part is that several of these overlap with Plan B’s own side effects. The difference is duration: Plan B side effects typically fade within a few days, while pregnancy symptoms persist and often intensify.
Two symptoms require immediate medical attention regardless of whether you think Plan B worked. Severe abdominal pain, especially if it’s sharp and on one side, can be a sign of an ectopic pregnancy, where a fertilized egg implants outside the uterus. Heavy vaginal bleeding that soaks through a pad every hour for two hours or more also warrants urgent care. Both of these are rare, but they’re serious.
The Short Version of Your Timeline
- Days 1 to 5: Side effects like nausea, spotting, and cramping are normal and not a sign of success or failure.
- Days 7 to 14: Watch for your period. If it arrives, Plan B worked.
- Day 14 or later: If your period hasn’t come, take a home pregnancy test. A result at this point is generally reliable.
- Day 21: If you tested earlier and got a negative result but still haven’t had your period, test again for the most accurate answer.
The waiting period can feel stressful, but most people who take Plan B within the recommended window do not become pregnant. Your period’s arrival is the confirmation you’re looking for.