Taking Plan B is not bad for you. It is a safe, well-studied medication with no known long-term health risks, and it does not affect your future fertility. What it can do is cause several days of uncomfortable but temporary side effects, and if you take it frequently, your menstrual cycle may become unpredictable for a while. Here’s what actually happens in your body and what to expect.
What Plan B Does to Your Body
Plan B delivers a large dose of a synthetic hormone your body already produces naturally during your menstrual cycle. That hormone surge works primarily by delaying or preventing ovulation, so an egg never gets released to be fertilized. It can also make it harder for sperm to reach an egg by thickening cervical mucus. It does not end an existing pregnancy.
Because you’re getting a concentrated burst of hormone all at once rather than the small, steady dose in a daily birth control pill, your body notices. That hormonal jolt is what causes the side effects most people experience in the days that follow.
Short-Term Side Effects
Most side effects start within a few hours and resolve within two to three days. The most commonly reported ones include:
- Nausea and vomiting. This is the side effect people notice first. If you vomit within two hours of taking the pill, you may need to take another dose.
- Headache and dizziness. These typically fade quickly and respond to over-the-counter pain relief.
- Fatigue. Feeling unusually tired for a day or two is normal.
- Lower abdominal cramps. Mild to moderate cramping is common. Severe lower abdominal pain three to five weeks later is rare but worth getting checked out, as it could signal an ectopic pregnancy.
- Breast tenderness. Similar to what many people feel before their period, this usually passes in a few days.
- Diarrhea. Less common, but it happens.
None of these side effects indicate anything harmful is happening inside your body. They’re simply your system adjusting to that temporary hormone spike.
How It Affects Your Next Period
Your first period after taking Plan B will probably look different than usual. It might arrive up to a week early or a week late, and the flow could be heavier or lighter than normal. Spotting between periods is also common. These changes only affect the cycle right after you take it. By the following month, your period should return to its typical pattern.
If your period is more than a week late, it’s worth taking a pregnancy test. Plan B is effective but not 100%, so a delayed period could occasionally mean the pill didn’t work.
What Happens if You Take It Multiple Times
There are no known long-term complications from taking Plan B more than once, even within the same cycle. It does not cause infertility, and repeated use poses no established health risks beyond the same short-term side effects you’d get from a single dose.
That said, taking it frequently can make your periods irregular and harder to predict, sometimes for several cycles in a row. The side effects also stack up, so you may spend more time feeling nauseous, tired, or crampy than you’d like. Plan B is also less effective at preventing pregnancy than daily birth control methods like the pill, an IUD, or an implant. It works well as a backup, but relying on it as your primary method means accepting both a higher failure rate and more disruptive side effects.
One important note: if you need emergency contraception again within five days of your last dose, stick with the same type. Mixing Plan B (levonorgestrel) with a different emergency contraceptive called ella can reduce the effectiveness of both.
Weight and Effectiveness
Some research has found that Plan B becomes less effective in people who weigh more than 165 pounds, with some studies showing up to double the risk of pregnancy compared to someone at a lower weight. Other studies have been less conclusive, and the evidence overall is mixed.
The FDA has not added a weight restriction to Plan B’s label, and reproductive health experts still recommend taking it regardless of your weight if you need it. If you weigh over 165 pounds and want the most effective emergency option, a copper IUD inserted within five days is the gold standard, preventing over 99% of pregnancies. Your doctor or a clinic can help you decide which option makes the most sense for your situation.
No Impact on Future Fertility
This is one of the most persistent concerns, and the answer is clear. Plan B does not harm your chances of getting pregnant in the future. The World Health Organization, Cleveland Clinic, and other major medical organizations all confirm that the hormones in emergency contraception leave your system quickly and have no lasting effect on your reproductive health. There is no delay in the return to fertility after taking it. If you want to become pregnant in a future cycle, Plan B will not stand in your way.