So, you took it. Maybe the condom broke, or maybe things just got a little carried away in the heat of the moment. Now you’re sitting there, staring at the empty gold and white box, wondering if every weird cramp or sudden mood swing is part of the deal. You’re likely asking about plan b side effects for how long they’ll stick around, and honestly, the answer isn’t a clean "one-size-fits-all" number.
It’s a massive dose of hormones. Specifically, 1.5 milligrams of levonorgestrel. To put that in perspective, that’s way more than what’s in a daily birth control pill. Your system is essentially getting a loud, hormonal shout to stop ovulation immediately. Naturally, your body is going to have some thoughts about that.
Most people feel the "peak" of the weirdness within the first 24 to 48 hours. But for some, the aftermath—especially the cycle changes—can linger for a full month.
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The immediate aftermath: The first 72 hours
The first few days are usually the rockiest. You might feel like you have a mild flu or just a really bad case of PMS. Nausea is the big one. About 25% of people who take Plan B feel like they might throw up. It’s common. If you actually do vomit within two hours of taking the pill, call a pharmacist immediately, because you probably didn't absorb the medication and might need another dose.
Dizziness and fatigue are also huge players here. You might find yourself wanting to nap at 2:00 PM when you're normally a high-energy person. Your head might throb. These aren't signs that something is "wrong" with you; it’s just the levonorgestrel working through your bloodstream. Usually, these physical symptoms—the nausea, the headaches, the breast tenderness—fade away within three days. If you're still feeling nauseous on day five, it's worth checking in with a doctor, though it's rarely a medical emergency.
Plan b side effects for how long: The cycle disruption
This is where things get annoying. While the nausea leaves quickly, your period is a different story.
Because Plan B works by delaying ovulation, it effectively pushes the "pause" button on your internal calendar. This means your next period might be early. It might be late. It might be heavier than usual or weirdly light. According to clinical data from Foundation Consumer Healthcare (the folks who make Plan B One-Step), a significant number of users see their next period within seven days of the expected date. But "expected" is a loose term when you've just dropped a hormone bomb in your system.
Spotting is the great deceiver
You might notice some bleeding a few days after taking the pill. This isn't your period. It’s breakthrough bleeding. It’s basically your uterine lining reacting to the sudden spike and then drop in hormones. It’s incredibly common, but it’s also the #1 cause of panic because people mistake it for implantation bleeding or a very early period.
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If you're wondering about plan b side effects for how long the spotting lasts, it's usually just a few days of light pink or brown discharge. If you're soaking through a pad an hour, that's not a side effect; that's a reason to go to the ER.
The mood swing factor
Don't ignore the mental side. Some people feel totally fine. Others feel like they’re trapped in a dark cloud for a week. Levonorgestrel can mess with your neurotransmitters temporarily. You might feel irritable, anxious, or just "off." This usually clears up once the hormone levels stabilize, which typically takes about four to seven days.
Why your weight or timing might change things
There's a lot of chatter online about Plan B and weight. It's a nuanced topic. Research, including studies cited by the CDC, suggests that levonorgestrel emergency contraception might be less effective for individuals with a BMI over 30. However, even if the effectiveness is slightly lower, the side effects can still be just as intense—sometimes more so if your metabolic rate processes the hormones differently.
Then there's the timing of your cycle. If you took Plan B right before you were supposed to ovulate, your body is fighting a harder battle to suppress that egg release. This can lead to more significant "rebound" symptoms when your body finally does ovulate later in the month.
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When should you actually worry?
Most of the time, you're fine. You’re just uncomfortable. But there are "red flag" symptoms that mean the "side effects" are actually something else.
Severe abdominal pain is the big one. While cramping is a normal Plan B side effect, sharp, one-sided pain in your lower belly could be a sign of an ectopic pregnancy (a pregnancy outside the uterus). While Plan B doesn't cause ectopic pregnancies, if the pill fails and you do get pregnant, there is a slightly higher risk of it being ectopic.
- Severe pelvic pain: Don't wait. Go to urgent care.
- Persistent vomiting: If you can't keep water down.
- Rash or hives: You might be allergic to an ingredient in the pill.
- Long-term absence of period: If your period is more than a week late, take a test.
Practical steps for the next 14 days
First, take a deep breath. Stress actually delays your period further, which creates a vicious cycle of "Am I pregnant or just stressed?" panic.
- Wait three weeks to test. Taking a pregnancy test the day after you take Plan B is a waste of money. There isn't enough hCG in your system yet. Wait 21 days after the unprotected sex for the most accurate result.
- Track the symptoms. Use a notes app. Write down when the nausea started and ended. This helps if you eventually need to talk to a doctor.
- Ginger and hydration. For the nausea in those first 48 hours, ginger tea or candies are lifesavers. Stay hydrated, as the hormone shift can make you feel slightly dehydrated and contribute to those headaches.
- Use backup protection. Plan B is not a "get out of jail free" card for the rest of the month. It only covers the specific instance of sex you took it for. In fact, you might be more fertile a week later if your ovulation was simply delayed rather than canceled.
The reality of plan b side effects for how long they last comes down to your body's sensitivity. For most, the physical "ill" feeling is gone in 72 hours. The cycle weirdness? Expect that to linger until you've had at least one full, normal period. If you reach the six-week mark and your cycle hasn't returned to its usual rhythm, that’s your cue to schedule an appointment with your OB-GYN to reset your baseline.