Most side effects from Plan B pass within 24 hours, and nearly all resolve within 48 hours. The exception is changes to your menstrual cycle, which can show up days or weeks later and take one full cycle to normalize. The active ingredient in Plan B has an elimination half-life of about 27.5 hours, meaning your body clears most of the drug within two to three days.
What the First 48 Hours Look Like
The most common side effects hit within a few hours of taking Plan B and feel a lot like premenstrual symptoms: nausea, lower abdominal cramping, fatigue, headache, dizziness, and breast tenderness. Nausea is the complaint most people notice first. For the majority of users, these symptoms fade within a single day. If any of them persist beyond 48 hours or feel severe, that’s worth a call to a healthcare provider.
Taking the pill with food or a small snack can take the edge off nausea. If you vomit within two hours of swallowing the tablet, your body may not have absorbed enough of the medication for it to work. In that case, you likely need to take another dose. Vomiting that happens more than two hours after the dose does not require a repeat pill.
Changes to Your Next Period
Your next period arriving up to a week early or a week late is normal after Plan B. This is the side effect that catches people off guard because it shows up well after the nausea and fatigue have faded. The hormonal surge from Plan B can shift the timing of ovulation, which pushes your period forward or back depending on where you were in your cycle when you took it.
You may also notice that the flow itself is different. Some people bleed more heavily than usual, others lighter. Spotting between periods is common too. These changes are generally limited to that one cycle. By your second period after taking Plan B, your cycle typically returns to its regular pattern. If you’ve taken Plan B more than once in a single menstrual cycle, flow changes are more likely and can be more pronounced.
A period that’s more than a week late is worth a pregnancy test, not because Plan B caused a problem, but because no emergency contraceptive is 100 percent effective.
How Quickly Your Body Clears the Drug
Plan B contains a single 1.5 mg dose of a synthetic hormone. According to FDA prescribing data, it has an elimination half-life of roughly 27.5 hours. That means about half the drug is gone after one day, three-quarters after two days, and the vast majority after three days. By day five, the amount remaining in your system is negligible. This timeline lines up with why most physical side effects resolve well within the first 48 hours.
No Lasting Impact on Fertility
Plan B does not affect your ability to get pregnant in the future. A 2022 systematic review analyzing 33 studies on the active ingredient in Plan B found no impact on fallopian tube function, no increased risk of ectopic pregnancy, no association with miscarriage or stillbirth, and no disruption to menstrual cycles beyond the immediate one. It is a one-time hormonal event, not a medication that accumulates or causes lasting changes to your reproductive system. Using it more than once does not change this.
Signs That Something Else Is Going On
Mild cramping after Plan B is expected. Severe abdominal pain is not. Sharp, intense pain on one side of your lower abdomen could signal an ectopic pregnancy, which is a medical emergency unrelated to Plan B itself. Bleeding that soaks through a pad every hour for two or more hours is another red flag that goes beyond a normal side effect. Persistent vomiting that prevents you from keeping the pill down also needs medical attention, since a provider can prescribe anti-nausea medication and help you figure out next steps for contraceptive coverage.