Most Plan B side effects are short-lived, typically resolving within 24 to 48 hours. The hormone in Plan B (levonorgestrel) has an average half-life of about 27.5 hours, meaning your body clears most of it within two to three days. Nausea, headache, fatigue, and dizziness tend to follow that same timeline. Changes to your menstrual cycle, however, can stretch out over several weeks.
What the First Few Days Feel Like
The most common side effects hit within the first several hours after taking Plan B. Nausea is the one people notice most, and it usually fades within a day. Headache, dizziness, breast tenderness, and fatigue follow a similar pattern. These symptoms track closely with the drug’s presence in your bloodstream: levonorgestrel peaks within a few hours and drops by half roughly every 27 hours. By day three, levels are low enough that most physical symptoms have disappeared entirely.
If you vomit within two hours of taking the pill, your body may not have absorbed enough of the medication for it to work. In that case, you likely need to take another dose. A pharmacist can help you figure out whether a second pill is necessary.
How Your Period May Change
Menstrual changes are the most common and longest-lasting effect of Plan B. Your next period might arrive earlier than expected, later than usual, or look different from what you’re used to. It could be heavier, lighter, or more spotty. These shifts can feel unsettling, but they’re a normal response to the sudden spike of synthetic hormone.
Research on over 200 women who took levonorgestrel emergency contraception found statistically significant changes in cycle length, period length, and menstrual appearance compared to their usual patterns. About 30% of users experience some bleeding within seven days of taking the pill, which can be confusing if you’re not expecting it. Up to 13% of users see their period delayed by more than a week. The timing of these changes depends partly on where you were in your cycle when you took the pill. If you took it well before ovulation, your cycle is more likely to shorten. If you took it close to or after ovulation, a delay is more common.
Plan B can delay your period by up to one week. If your period is more than a week late, a pregnancy test is a reasonable next step.
Spotting Between Periods
Unscheduled spotting or light bleeding in the days after taking Plan B is common and not a sign that anything is wrong. In one study, about 15% of participants experienced new spotting between periods that they hadn’t had before. This intermenstrual bleeding typically resolves on its own within a few days and doesn’t require treatment. It’s caused by the hormonal disruption to your uterine lining, not by implantation or pregnancy.
What Happens With Repeated Use
Using Plan B once won’t throw your cycle off for months. But using it frequently can make your periods increasingly irregular and harder to predict. There’s no medical danger to taking it more than once, but the cumulative hormonal disruption can make cycle tracking unreliable for a while. If you find yourself reaching for emergency contraception regularly, a daily or long-acting contraceptive method will be both more effective and easier on your cycle.
Weight and Effectiveness
Plan B’s effectiveness drops significantly at higher body weights. Research shows the drug begins losing efficacy around 154 pounds (70 kg) and may have essentially no effect at 176 pounds (80 kg) and above. People with a BMI of 30 or higher had roughly four times the risk of pregnancy compared to those with a BMI under 25. This isn’t about side effects lasting longer or shorter. It’s about the drug not reaching high enough concentrations to work reliably. If you weigh more than 155 pounds, a copper IUD or a different emergency contraceptive pill that uses a different hormone may be a better option.
Symptoms That Aren’t Normal Side Effects
Most discomfort after Plan B is mild and predictable. But certain symptoms warrant attention because they can signal something unrelated to the pill itself. Severe pain on one side of your lower abdomen, especially combined with vaginal spotting and vomiting, can look similar to common Plan B side effects but may instead indicate an ectopic pregnancy, where a fertilized egg implants outside the uterus. This is a medical emergency.
The key difference is intensity and location. Plan B side effects are generally diffuse: mild nausea, a dull headache, overall fatigue. Sharp, localized pelvic pain that worsens with movement is not a typical reaction to the pill. If you experience that kind of pain in the weeks after taking Plan B, especially if a pregnancy test comes back positive, seek medical care promptly.