Plan B works by delivering a large dose of a hormone that delays or prevents ovulation, stopping an egg from being released so sperm can’t fertilize it. Most people notice side effects within the first day or two, and the whole experience typically resolves within a week. Here’s what to expect in your body, when to take a pregnancy test, and what to do about birth control going forward.
How Plan B Actually Works
Plan B contains a synthetic version of progesterone, a hormone your body already produces. The pill floods your system with enough of it to pause or delay ovulation. If no egg is released, pregnancy can’t happen. It does not end an existing pregnancy or harm a developing embryo. If a fertilized egg has already implanted, Plan B will have no effect.
This is why timing matters so much. Taken within the first 24 hours after unprotected sex, Plan B prevents about 95% of expected pregnancies. Effectiveness drops with every passing hour, and the pill is generally not effective beyond 72 hours. The sooner you take it, the better it works.
Side Effects in the First Few Days
Plan B pushes a significant amount of hormone into your system at once, so side effects are common and usually start within hours. Nausea is the most frequent, affecting up to 23% of users. Fatigue hits about 17%, and headaches and dizziness each affect roughly 11 to 17%. You may also notice breast tenderness, lower abdominal or pelvic pain, or light spotting.
These side effects are temporary. Nausea usually passes within a day or two. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose. Fatigue and headaches typically clear up within 24 to 48 hours. None of these effects are dangerous on their own, but they can be uncomfortable enough to disrupt your day.
What Happens to Your Period
Plan B almost always affects your next period. Your cycle may arrive a week early, several days late, or right on time. The bleeding itself can be heavier or lighter than usual, and some people experience spotting between their period and when they took the pill. All of this is normal and results from the hormonal disruption caused by the large dose of progesterone.
If your period is more than a week late, that’s when a pregnancy test becomes important. A delay of a few days is expected and not a sign that Plan B failed.
When to Take a Pregnancy Test
A pregnancy test won’t give you a reliable answer right away. The most accurate approach is to wait until the first day of your missed period. If your cycle is irregular or you’re not sure when your period is due, wait at least 21 days after the unprotected sex to test. Testing earlier than this can produce a false negative because your body may not have produced enough pregnancy hormone to detect.
If the test is negative but your period still hasn’t come after another week, test again. Stress, hormonal shifts from the pill itself, and normal cycle variation can all delay things.
Factors That Reduce Effectiveness
Body weight plays a role. Research has found that Plan B becomes less effective for people with a BMI above 26. If you’re in that range, a copper IUD inserted as emergency contraception is a more reliable option, or you can ask a provider about a different emergency contraceptive pill that works better at higher body weights.
Certain medications also interfere with how your body processes Plan B. Drugs used to treat epilepsy, tuberculosis, and HIV can speed up the breakdown of the active ingredient, reducing its concentration in your blood by as much as 50%. The herbal supplement St. John’s wort has the same effect. This interaction can persist for up to four weeks after you stop taking the interfering medication, so even recently discontinued drugs can be an issue.
Restarting Birth Control After Plan B
You can restart or begin hormonal birth control (the pill, patch, ring, implant, or injection) immediately after taking Plan B. There’s no need to wait for your next period. If you were already on the pill and missed doses that led you to take Plan B, you can continue your current pack where you left off. Patch users should start a fresh patch.
The key detail: use a backup method like condoms for the first seven days after restarting your hormonal method. During that window, the birth control hasn’t had enough time to reliably suppress ovulation on its own. A copper IUD is the one exception. It can be inserted the same day you take Plan B and works immediately, with no backup needed.
If you took a different type of emergency contraceptive (the one sold as ella, which contains a different active ingredient), the rules change. You need to wait six days before starting any hormonal birth control containing progesterone, because the two drugs can interfere with each other’s effectiveness. A backup method is needed from the time you take it until seven days into your hormonal method.
Severe Symptoms Worth Watching For
Plan B’s common side effects are mild and short-lived, but severe abdominal pain, especially if it’s one-sided and paired with vaginal spotting or vomiting, can signal an ectopic pregnancy (a pregnancy developing outside the uterus). This is rare, but it’s a medical emergency. Plan B does not cause ectopic pregnancies, but if the pill fails and a pregnancy occurs, there’s a small chance it could implant in the wrong place. Sharp, worsening pain in the weeks after taking Plan B warrants immediate medical evaluation.