Most side effects from Plan B last one to two days. The active ingredient has a half-life of about 27.5 hours, meaning your body eliminates half of it roughly every day. Within two to three days, the drug is largely cleared from your system, and physical symptoms like nausea, headache, and fatigue typically fade on that same timeline. The exception is changes to your menstrual cycle, which can linger for several weeks.
The Most Common Side Effects
In a clinical trial of 1,359 women who took Plan B One-Step, the side effects reported most frequently were:
- Heavier menstrual bleeding: 30.9%
- Nausea: 13.7%
- Lower abdominal pain: 13.3%
- Fatigue: 13.3%
- Headache: 10.3%
- Dizziness: 9.6%
- Breast tenderness: 8.2%
- Period delayed more than 7 days: 4.5%
Nausea, fatigue, headache, and dizziness are the side effects most tied to the drug itself circulating in your body. Because the medication’s half-life is about 27.5 hours, these symptoms generally peak within the first 24 hours and resolve within 48 hours. Most women feel back to normal within a day or two of taking the pill.
How Plan B Affects Your Period
Bleeding changes are the most common and longest-lasting effect. Nearly a third of women in clinical trials experienced heavier menstrual bleeding, and about 31% noticed some shift in their bleeding pattern overall. More than half had their period arrive within two days of when they expected it, but for others, the disruption was more noticeable.
Your period may come early, come late, or be heavier or lighter than usual. Plan B can delay your next period by up to one week. About 4.5% of women in the clinical trial had their period show up more than seven days late. This happens because the large dose of hormone can temporarily shift the timing of ovulation and the buildup of your uterine lining.
These cycle changes are limited to the current cycle and possibly the one after it. They do not carry over into future months. If your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test.
Why Some Side Effects Fade Faster Than Others
The short-lived symptoms, like nausea and dizziness, are a direct response to a sudden spike of synthetic hormone in your bloodstream. As your body metabolizes the drug over 24 to 48 hours, those symptoms drop off. Think of it like a wave that crests and then falls.
Menstrual changes work differently. The hormone doesn’t just pass through your system; it intervenes in a cycle that unfolds over weeks. It can delay the release of an egg, alter the uterine lining, or shift hormone levels that your body then needs time to recalibrate. That’s why bleeding irregularities can persist for the rest of that cycle even though the drug itself is long gone.
Managing Nausea and Discomfort
Nausea is the side effect most likely to make you miserable in the first few hours. Taking Plan B with food can help settle your stomach. If you’ve taken emergency contraception before and know nausea is a problem for you, an over-the-counter anti-nausea medication taken 30 minutes to an hour before the pill can reduce it.
One important rule: if you vomit within two hours of swallowing the pill, it may not have been fully absorbed, and you should take another dose. For the headache and fatigue, standard over-the-counter pain relievers and rest are usually enough. These symptoms rarely last beyond the first day.
No Long-Term Effects on Fertility or Health
Plan B does not affect your future fertility. There is no delay in the return to fertility after taking it. The World Health Organization notes that even repeated use of emergency contraception poses no known long-term health risks, though using it frequently can increase short-term side effects like menstrual irregularity. It is not designed as a routine contraceptive, but occasional repeat use is safe.
Emergency contraception also does not cause or increase the risk of ectopic pregnancy. It actually lowers that risk by preventing pregnancy in the first place.
Signs That Need Medical Attention
Normal Plan B side effects are mild and self-limiting. A few specific symptoms fall outside that range. Severe abdominal pain, especially sharp or one-sided pain, can be a sign of an ectopic pregnancy (a pregnancy outside the uterus) and needs prompt evaluation. Bleeding heavy enough to soak through a pad every hour for two hours or longer is also not a typical side effect and warrants a call to your doctor. And if your period hasn’t come within three weeks of taking the pill, a pregnancy test is the logical next step.