Plan B’s most common side effects are nausea, headache, and changes to your next period. These effects are temporary, typically lasting only a few days, and they don’t pose any long-term health risks. Here’s what to expect and what’s worth paying attention to.
The Most Common Side Effects
Plan B delivers a large dose of a synthetic hormone that temporarily disrupts your normal cycle to prevent pregnancy. That hormonal surge is what causes the short-term side effects most people experience. The most frequently reported ones include nausea, headache, fatigue, dizziness, breast tenderness, and lower abdominal pain.
These side effects are generally mild. If nausea is a concern, eating a small meal before or right after taking the pill can help. Some people also experience vomiting. If you throw up 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 Period
This is the side effect that catches people off guard the most. Plan B can shift the timing, flow, and feel of your next period in ways that mimic early pregnancy symptoms, which understandably causes anxiety.
Your period may arrive up to a week later than expected. It may also come a few days early. When it does show up, it can be heavier or lighter than usual. Some people notice light spotting or bleeding between periods in the days after taking the pill, even before their period is due. All of this is normal and results from the temporary hormonal disruption.
If your period is more than a week late, that’s worth following up on with a pregnancy test. A late period alone doesn’t mean Plan B failed, but confirming is straightforward and will give you a clear answer.
How Long Side Effects Last
Most acute side effects, like nausea, headache, and dizziness, resolve within 24 to 48 hours. Period-related changes can persist through your next full cycle, but things typically return to normal by the cycle after that. There’s no lingering hormonal effect beyond that window.
No Impact on Future Fertility
Plan B works by delaying ovulation so that sperm and egg never meet. A comprehensive review published in the journal Contraception found no evidence that the drug affects implantation or alters the uterine lining in ways that would interfere with future pregnancies. Multiple studies examining tissue-level markers of fertility found no significant changes compared to controls. In short, taking Plan B, even more than once, does not reduce your ability to get pregnant later.
Plan B is also not an abortion pill. If a fertilized egg has already implanted, the drug won’t disrupt the pregnancy.
Weight and Effectiveness
Plan B becomes less effective if you weigh more than 165 pounds. This isn’t a side effect in the traditional sense, but it’s critical information that directly affects whether the pill will work for you. If you’re above that weight threshold, a copper IUD placed within five days of unprotected sex is the most reliable emergency contraception option, and your provider can help you access one quickly.
Medications That Reduce Effectiveness
Certain medications speed up how quickly your body breaks down Plan B’s active ingredient, which can cut its effectiveness significantly. The UK’s medicines safety agency has flagged several categories:
- Epilepsy medications such as carbamazepine, phenytoin, and barbiturates
- Tuberculosis medications such as rifampicin
- HIV medications such as efavirenz, which can reduce blood levels of Plan B’s active ingredient by roughly 50%
- Antifungal medications such as griseofulvin
- St. John’s wort, a common herbal supplement for mood
Even after stopping one of these medications, the effect on your body’s processing speed can persist for up to four weeks. If you take any of these, a copper IUD is a more reliable emergency option.
Safety While Breastfeeding
Plan B does pass into breast milk, but only in minimal quantities. CDC safety data shows that breastfeeding outcomes did not differ between women who took the drug and those who didn’t. Neither the CDC nor WHO flags any special safety concerns for breastfeeding mothers using emergency contraception.
Repeated Use
Taking Plan B more than once does not cause cumulative harm or build up in your system. Each dose is a one-time hormonal event that your body processes and clears. That said, repeated use can make your periods unpredictable for a stretch, simply because each dose temporarily disrupts your cycle’s timing. If you find yourself reaching for Plan B frequently, a regular contraceptive method will be both more effective at preventing pregnancy and easier on your cycle’s regularity.