Most Plan B side effects resolve within 24 hours. The most common reactions, including nausea, headache, and fatigue, are mild and short-lived. If any symptoms linger past 48 hours, that’s unusual and worth a call to a healthcare provider.
What the First 24 Hours Look Like
Plan B contains a single large dose of a hormone your body already produces in smaller amounts during your normal cycle. That sudden spike is what triggers side effects, and it’s also why they fade quickly as your body processes the hormone.
The most commonly reported effects include nausea, headache, dizziness, fatigue, lower abdominal cramps, and breast tenderness. For most people, these start within a few hours of taking the pill and clear up by the next day. By day three, the vast majority of people feel completely back to normal.
Nausea is the side effect people worry about most. It tends to be mild, more like a queasy stomach than the kind that makes you vomit. Taking Plan B with food or a small snack can help. If you do vomit within two hours of swallowing the pill, the dose may not have been fully absorbed. In that case, talk to a pharmacist about whether you need to take another one.
Spotting and Light Bleeding
Some people notice light spotting in the days after taking Plan B. This is not a period. It typically looks like a few spots of blood on toilet paper or underwear, much lighter than a normal menstrual flow. It can last a day or two, though for some people it sticks around a bit longer. You won’t need anything more than a light pad or liner.
This spotting happens because the hormone dose can temporarily affect the lining of your uterus. It’s a normal response and not a sign that anything is wrong.
Changes to Your Next Period
The side effect that lasts the longest isn’t pain or nausea. It’s the disruption to your menstrual cycle. Plan B can delay your next period by up to a week. It can also make your period arrive a few days early, or cause it to be heavier or lighter than usual.
Where you are in your cycle when you take Plan B plays a role. If you take it early in your cycle (before ovulation), your period is more likely to come early. If you take it later, a delay is more common. Either way, your cycle typically returns to its normal pattern by the following month. If your period is more than a week late, a pregnancy test is a reasonable next step.
Effects on Future Fertility
Plan B does not affect your ability to get pregnant in the future. A review published in the journal Contraception examined the evidence and found no detrimental effect on fertility from levonorgestrel-based contraceptives, regardless of how many times they were used. Conception rates after use matched those of the general population. The same review also found no increased risk of miscarriage, fetal abnormalities, or developmental problems in pregnancies that occurred after emergency contraception exposure.
Weight and Effectiveness
Side effects aren’t the only thing worth knowing about. Plan B may be less effective if you weigh more. Evidence reviewed by the American College of Obstetricians and Gynecologists indicates that levonorgestrel-based emergency contraception loses some efficacy for people with a BMI of 25 or higher. That doesn’t mean it won’t work at all, and no one should be discouraged from taking it based on weight alone. But if this applies to you, a copper IUD is the most effective form of emergency contraception at any weight, and a prescription alternative called ulipristal acetate also works better across a wider weight range.
Symptoms That Aren’t Normal
Mild cramps, a headache, and some queasiness are expected. Severe abdominal or pelvic pain is not. If you experience intense pain along with vaginal bleeding, extreme dizziness or fainting, or pain in your shoulder, these can be signs of an ectopic pregnancy, a condition where a fertilized egg implants outside the uterus. This is a medical emergency unrelated to Plan B itself, but it’s important to recognize because someone taking emergency contraception is already aware that pregnancy is a possibility.
Outside of that rare scenario, the bottom line is straightforward: most physical side effects wrap up within a day, spotting may last a few days, and your period should normalize within one cycle.