Most Plan B side effects are short-lived, typically resolving within 24 to 48 hours after taking the pill. The most common symptoms are nausea, dizziness, and headache, and they tend to feel similar to mild premenstrual discomfort. The one effect that can linger longer is a shift in your menstrual cycle, which may take a few weeks to fully sort itself out.
The Most Common Side Effects
Plan B delivers a large, single dose of a synthetic hormone that works primarily by delaying ovulation. That hormonal surge is what causes most of the side effects you feel. In clinical trials, about 23% of women experienced nausea, 11% felt dizzy, and roughly 6% actually vomited. Headaches, fatigue, and breast tenderness also show up frequently. These symptoms generally start within a few hours of taking the pill and fade within a day or two.
Nausea is the side effect people worry about most. If you take Plan B with food or right before bed, you can minimize how much it bothers you. If you vomit within two hours of swallowing the pill, it may not have been fully absorbed, and you may need another dose.
How Plan B Affects Your Next Period
This is the side effect with the longest timeline, and it’s the one that causes the most anxiety. Plan B can make your next period come early, come late, or be heavier or lighter than usual. Clinical data show that most women get their period within two days to one week of when they expected it. So if your period was due on a Friday, it could arrive anywhere from the Wednesday before to the following Friday.
Some women also experience spotting or light bleeding in the days after taking the pill. This is not a period. It’s a response to the sudden hormonal shift and usually stops on its own within a couple of days. The bleeding can be confusing because it might look like an early period or, for some, raise concerns about implantation bleeding. The simplest way to get clarity is with a pregnancy test, but you need to wait long enough for it to be accurate.
If your period is more than a week late, take a pregnancy test. Planned Parenthood recommends testing three weeks after taking Plan B for the most reliable result. Plan B is roughly 87% effective when taken within 72 hours of unprotected sex, so a late period does not necessarily mean it failed, but it’s worth confirming.
Day-by-Day Breakdown
Here’s a general sense of what to expect in the days and weeks after taking Plan B:
- First 24 hours: Nausea, dizziness, headache, and fatigue are most likely during this window. These are the peak side effects and the ones that resolve fastest.
- Days 2 to 3: Most physical symptoms like nausea and dizziness have faded. You may notice breast tenderness or mild cramping that lingers slightly longer.
- Days 3 to 7: Spotting or irregular bleeding can appear. This is normal and usually light.
- Weeks 1 to 4: Your menstrual cycle may be off. Your next period could arrive early or late, and the flow might be different than usual. By your second cycle after taking Plan B, things typically return to your normal pattern.
Symptoms That Are Not Normal
Plan B side effects should feel mild and manageable. Certain symptoms, however, are not caused by Plan B and need immediate attention. Severe lower abdominal or pelvic pain, especially on one side, combined with vaginal bleeding can be a sign of an ectopic pregnancy, where a fertilized egg implants outside the uterus. Shoulder pain, extreme lightheadedness, or fainting are signs the situation may be escalating and require emergency care. These symptoms are rare, but they are serious regardless of whether you’ve recently taken emergency contraception.
Repeated Use and Longer Side Effects
Taking Plan B more than once in the same cycle is safe, but it increases the likelihood and intensity of side effects. You’re essentially giving your body repeated large hormonal doses in a short window, so nausea and cycle disruption tend to be more pronounced. Your period may be significantly delayed or irregular for the next cycle or two. The pill remains effective each time, but it is less reliable than ongoing contraception methods, and the side effects stack up in a way that makes repeated use uncomfortable for most people.
If you find yourself needing emergency contraception frequently, a longer-acting option like a copper IUD (which also works as emergency contraception if placed within five days) or a hormonal birth control method will be more effective and cause fewer disruptive symptoms over time.