Most Plan B side effects resolve within 24 to 48 hours. The pill delivers a large single dose of a synthetic hormone that your body processes relatively quickly, with a half-life of roughly 24 to 27 hours. That means the drug is mostly cleared from your system within two to three days, and the short-term side effects follow the same timeline. Changes to your menstrual cycle, however, can linger into the following month.
What the First 48 Hours Look Like
The most common side effects hit within the first day or two after taking Plan B. Nausea is the one people notice most, and it can range from mild queasiness to feeling genuinely sick. Headache, fatigue, dizziness, and breast tenderness are also typical during this window. For the majority of people, these symptoms fade by day two or three. If you vomit within two hours of taking the pill, your body may not have absorbed enough of the medication, and you should take another dose.
Some people feel essentially normal within a few hours of taking the pill. Others feel off for a full day or two. Both experiences are common, and the intensity tends to depend on individual sensitivity to hormonal shifts. If nausea, vomiting, or any other symptom is still severe after 48 hours, that warrants a call to your doctor.
Spotting and Unexpected Bleeding
Light spotting or irregular bleeding in the days or weeks after taking Plan B is one of the most common effects, and it catches a lot of people off guard because it doesn’t follow a predictable schedule. This bleeding isn’t a period. It’s a response to the sudden surge and then drop in hormone levels. It can show up a few days after you take the pill or closer to when your period would normally arrive, and it may be lighter or heavier than what you’re used to.
This spotting is not dangerous, but it can make it harder to tell whether your next actual period has started. If you’re unsure whether bleeding you’re seeing is spotting or a true period, tracking the timing and flow for a few days usually clarifies things.
How Your Next Period May Change
This is the side effect with the longest tail. Plan B can shift the timing of your next period by several days in either direction. In one study of women who took levonorgestrel emergency contraception, about 21% had their cycle shorten by two or more days, while 24% had it lengthen by two or more days. Up to 13% experienced a delay of more than a week.
A period that arrives a few days early or a few days late is a normal response to the pill and not a sign of pregnancy. However, if your period is more than five days late, a pregnancy test is a reasonable next step. That’s the threshold recommended in the product’s own labeling. Your cycle typically returns to its regular pattern by the month after you take the pill.
The flow of your next period can also look different. Some people report heavier bleeding than usual, while others have a lighter period. These changes are temporary and limited to the cycle in which you took Plan B.
Why Side Effects Happen
Plan B contains 1.5 mg of levonorgestrel, a synthetic version of progesterone. That’s a much larger dose than what’s in a daily birth control pill, delivered all at once. Your body responds to this hormonal spike the same way it responds to other sudden hormonal shifts: nausea, fatigue, breast tenderness, and mood changes. As your liver metabolizes the drug over the next one to three days, hormone levels return to baseline and the symptoms ease.
The menstrual changes take longer to resolve because the hormone surge can delay or accelerate ovulation, which ripples forward through the rest of your cycle. Once your body completes that disrupted cycle and starts the next one, things typically reset.
Symptoms That Need Attention
Severe abdominal pain after taking Plan B is not a normal side effect. It can be a sign of an ectopic pregnancy, where a fertilized egg implants outside the uterus. This is a medical emergency regardless of whether you’ve taken emergency contraception. Very heavy bleeding that soaks through a pad in an hour or less, or severe nausea and vomiting that won’t let up, also warrant a call to your healthcare provider.
For the vast majority of people, Plan B’s side effects are mild, short-lived, and fully resolved within a few days to one menstrual cycle. The acute symptoms (nausea, headache, fatigue, breast tenderness) clear within 24 to 48 hours. The cycle-related effects (spotting, a late or early period, heavier flow) resolve by the start of your next normal period.